Mostly logic of when to send an email

Split out BaseMediaResource into MediaRepository

This is so that a single MediaRepository can be shared across all
resources, rather than having a "copy" per resource.

In particular this allows us to guard against both the thumbnail and
download resource triggering a download of remote content at the same
time.

Create log context in Measure if one doesn't exist

Lower timeout for make_membership_event

Simplify _check_password

fix cyrillic URL previews by hardcoding all page decoding to UTF-8 for now, rather than relying on lxml's heuristics which seem to get it wrong

Calls to make_membership_event are done in response to client requests,
and so should not be retried over long timeframes.

Use SynapseError 504 for Timeout errors

Fix login to error for nonexistent users

Fixes SYN-680

Make v2_alpha reg follow the AS API specification

The spec is clear the key should be 'user' not 'username' and this is indeed
the case for v1. This is not true for v2_alpha though, which is what this
commit is fixing.

comment out 2c838f64 due to risk of https://en.wikipedia.org/wiki/Billion_laughs attacks - thanks @torhve

Measure push action generator

Don't push for everyone who ever sent an RR to the room

Add push index. Add extra Measure

Sanitize the optional dependencies for spider API

Only return unread notifications

Make get_unread_push_actions_for_user_in_range only return unread event actions, being more true to its name. Done in two separate sql queries to get actions after a read receipt and those in a room wiht no receipt at all. SQL queries by Erik.

Don't return empty public rooms