Standardise the module interface (#10062)

This PR adds a common configuration section for all modules (see docs). These modules are then loaded at startup by the homeserver. Modules register their hooks and web resources using the new `register_[...]_callbacks` and `register_web_resource` methods of the module API.

docs/sample_config.yaml

@@ -31,6 +31,22 @@
 #
 # [1] https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html
 
+
+## Modules ##
+
+# Server admins can expand Synapse's functionality with external modules.
+#
+# See https://matrix-org.github.io/synapse/develop/modules.html for more
+# documentation on how to configure or create custom modules for Synapse.
+#
+modules:
+    # - module: my_super_module.MySuperClass
+    #   config:
+    #       do_thing: true
+    # - module: my_other_super_module.SomeClass
+    #   config: {}
+
+
 ## Server ##
 
 # The public-facing domain of the server
@@ -2491,19 +2507,6 @@ push:
   #group_unread_count_by_room: false
 
 
-# Spam checkers are third-party modules that can block specific actions
-# of local users, such as creating rooms and registering undesirable
-# usernames, as well as remote users by redacting incoming events.
-#
-spam_checker:
-   #- module: "my_custom_project.SuperSpamChecker"
-   #  config:
-   #    example_option: 'things'
-   #- module: "some_other_project.BadEventStopper"
-   #  config:
-   #    example_stop_events_from: ['@bad:example.com']
-
-
 ## Rooms ##
 
 # Controls whether locally-created rooms should be end-to-end encrypted by