Fix wrong avatars when inviting multiple users when creating room

Add all local users to the user_directory and optionally search them

This was introduced in d7ea8c48 / PR #2728

support custom login types for validating users

Wire the custom login type support from password providers into the UI-auth
user-validation flows.

Factor out a validate_user_via_ui_auth method

Collect together all the places that validate a logged-in user via UI auth.

Refactor UI auth implementation

Instead of returning False when auth is incomplete, throw an exception which
can be caught with a wrapper.

federation_client script: Support for posting content

Improve comments on get_user_by_access_token

Delete devices and pushers on logouts etc

Initial commit; this doesn't work yet - the LIKE filtering seems too aggressive.
It also needs _do_initial_spam to be aware of prepopulating the whole user_directory_search table with all users...
...and it needs a handle_user_signup() or something to be added so that new signups get incrementally added to the table too.

Committing it here as a WIP

Make sure that we delete devices whenever a user is logged out due to any of
the following situations:

 * /logout
 * /logout_all
 * change password
 * deactivate account (by the user or by an admin)
 * invalidate access token from a dynamic module

Fixes #2672.

Non-functional refactoring to move set_password. This means that we'll be able
to properly deactivate devices and access tokens without introducing a
dependency loop.

Non-functional refactoring to move deactivate_account. This means that we'll be
able to properly deactivate devices and access tokens without introducing a
dependency loop.

Whenever an access token is invalidated, we should remove the associated
pushers.

Clear logcontext before starting fed txn queue runner

Fix 500 when joining matrix-dev

matrix-dev has an event (`$/6ANj/9QWQyd71N6DpRQPf+SDUu11+HVMeKSpMzBCwM:zemos.net`)
which has no `hashes` member.

Check for missing `hashes` element in events.

because I have to reverse-engineer this every time.

These processes take a long time compared to the request, so there is lots of
"Entering|Restoring dead context" in the logs. Let's try to shut it up a bit.