We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

Implement group join API

Add response size metrics

postgres port script: fix state_groups_pkey error

as opposed to join_policy, which is really only pertinent to the
synapse implementation of the group server.

By doing this we keep the group server concept extensible by
allowing arbitrarily complex rules for deciding whether a group
is openly joinable.

Fixes https://github.com/matrix-org/synapse/issues/3050.

Improve logging and comments. Group all the stuff to do with inspecting tables
together rather than creating the port tables in the middle.

We really shouldn't spit out "Failed to create port table", it looks scary.

use PUT instead of POST for federating groups/m.join_policy

Add a blurb explaining the main synapse worker

Add room_id to the response of `rooms/{roomId}/join`

Improve database cache performance

Remove redundant metrics which were deprecated in 0.27.0.

Fixes an issue where a cache invalidation would invalidate *all* pending
entries, rather than just the entry that we intended to invalidate.

Add joinability for groups

R30 stats

Remove uses of events.content

Use iteritems over item to loop over dict
formatting

phone home cache size configurations