Add delete room admin endpoint (#7613)

The Delete Room admin API allows server admins to remove rooms from server
and block these rooms.
`DELETE /_synapse/admin/v1/rooms/<room_id>`
It is a combination and improvement of "[Shutdown room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/shutdown_room.md)" and "[Purge room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/purge_room.md)" API.

Fixes: #6425 

It also fixes a bug in [synapse/storage/data_stores/main/room.py](synapse/storage/data_stores/main/room.py) in ` get_room_with_stats`.
It should return `None` if the room is unknown. But it returns an `IndexError`.
901b1fa561/synapse/storage/data_stores/main/room.py (L99-L105)

Related to:
- #5575
- https://github.com/Awesome-Technologies/synapse-admin/issues/17

Signed-off-by: Dirk Klimpel dirk@klimpel.org

docs/admin_api/purge_room.md

@@ -5,6 +5,8 @@ This API will remove all trace of a room from your database.
 
 All local users must have left the room before it can be removed.
 
+See also: [Delete Room API](rooms.md#delete-room-api)
+
 The API is:
 
 ```

docs/admin_api/rooms.md

@@ -318,3 +318,97 @@ Response:
   "state_events": 93534
 }
 ```
+
+# Delete Room API
+
+The Delete Room admin API allows server admins to remove rooms from server
+and block these rooms.
+It is a combination and improvement of "[Shutdown room](shutdown_room.md)" 
+and "[Purge room](purge_room.md)" API.
+
+Shuts down a room. Moves all local users and room aliases automatically to a
+new room if `new_room_user_id` is set. Otherwise local users only
+leave the room without any information.
+
+The new room will be created with the user specified by the `new_room_user_id` parameter
+as room administrator and will contain a message explaining what happened. Users invited
+to the new room will have power level `-10` by default, and thus be unable to speak.
+
+If `block` is `True` it prevents new joins to the old room.
+
+This API will remove all trace of the old room from your database after removing
+all local users.
+Depending on the amount of history being purged a call to the API may take
+several minutes or longer.
+
+The local server will only have the power to move local user and room aliases to
+the new room. Users on other servers will be unaffected.
+
+The API is:
+
+```json
+POST /_synapse/admin/v1/rooms/<room_id>/delete
+```
+
+with a body of:
+```json
+{
+    "new_room_user_id": "@someuser:example.com",
+    "room_name": "Content Violation Notification",
+    "message": "Bad Room has been shutdown due to content violations on this server. Please review our Terms of Service.",
+    "block": true
+}
+```
+
+To use it, you will need to authenticate by providing an ``access_token`` for a
+server admin: see [README.rst](README.rst).
+
+A response body like the following is returned:
+
+```json
+{
+    "kicked_users": [
+        "@foobar:example.com"
+    ],
+    "failed_to_kick_users": [],
+    "local_aliases": [
+        "#badroom:example.com",
+        "#evilsaloon:example.com"
+    ],
+    "new_room_id": "!newroomid:example.com"
+}
+```
+
+## Parameters
+
+The following parameters should be set in the URL:
+
+* `room_id` - The ID of the room.
+
+The following JSON body parameters are available:
+
+* `new_room_user_id` - Optional. If set, a new room will be created with this user ID
+      as the creator and admin, and all users in the old room will be moved into that
+      room. If not set, no new room will be created and the users will just be removed
+      from the old room. The user ID must be on the local server, but does not necessarily
+      have to belong to a registered user.
+* `room_name` - Optional. A string representing the name of the room that new users will be
+                invited to. Defaults to `Content Violation Notification`
+* `message` - Optional. A string containing the first message that will be sent as
+              `new_room_user_id` in the new room. Ideally this will clearly convey why the
+               original room was shut down. Defaults to `Sharing illegal content on this server
+               is not permitted and rooms in violation will be blocked.`
+* `block` - Optional. If set to `true`, this room will be added to a blocking list, preventing future attempts to
+  join the room. Defaults to `false`.
+
+The JSON body must not be empty. The body must be at least `{}`.
+
+## Response
+
+The following fields are returned in the JSON response body:
+
+* `kicked_users` - An array of users (`user_id`) that were kicked.
+* `failed_to_kick_users` - An array of users (`user_id`) that that were not kicked.
+* `local_aliases` - An array of strings representing the local aliases that were migrated from
+                    the old room to the new.
+* `new_room_id` - A string representing the room ID of the new room.

docs/admin_api/shutdown_room.md

@@ -10,6 +10,8 @@ disallow any further invites or joins.
 The local server will only have the power to move local user and room aliases to
 the new room. Users on other servers will be unaffected.
 
+See also: [Delete Room API](rooms.md#delete-room-api)
+
 ## API
 
 You will need to authenticate with an access token for an admin user.