Add ability for access tokens to belong to one user but grant access to another user. (#8616)

We do it this way round so that only the "owner" can delete the access token (i.e. `/logout/all` by the "owner" also deletes that token, but `/logout/all` by the "target user" doesn't).

A future PR will add an API for creating such a token.

When the target user and authenticated entity are different the `Processed request` log line will be logged with a: `{@admin:server as @bob:server} ...`. I'm not convinced by that format (especially since it adds spaces in there, making it harder to use `cut -d ' '` to chop off the start of log lines). Suggestions welcome.

synapse/appservice/__init__.py

@@ -52,11 +52,11 @@ class ApplicationService:
         self,
         token,
         hostname,
+        id,
+        sender,
         url=None,
         namespaces=None,
         hs_token=None,
-        sender=None,
-        id=None,
         protocols=None,
         rate_limited=True,
         ip_range_whitelist=None,