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.

Add get endpoint for pushers

Decodes pushers rows on the main thread rather than the db thread and uses _simple_select_list. Also do the same to the function I copied and factor out the duplication into a helper function.

Check if we've already backfilled events

As per https://github.com/matrix-org/matrix-doc/pull/308

Change pushers to use the event_actions table

and wrap unsafe process in a try block

URL previewing support

Add url_preview_ip_range_blacklist to let admins specify internal IP ranges that must not be spidered.
Add url_preview_url_blacklist to let admins specify URL patterns that must not be spidered.
Implement a custom SpiderEndpoint and associated support classes to implement url_preview_ip_range_blacklist
Add commentary and generally address PR feedback

 * If the event is an invite event, add the invitee to list of user we run push rules for (if they have a pusher etc)
 * Move invite_for_me to be higher prio than member events otherwise member events matches them
 * Spell override right

Read from DNS cache if within TTL