The API is now under
 /groups/$group_id/setting/m.join_policy

and expects a JSON blob of the shape

```json
{
  "m.join_policy": {
    "type": "invite"
  }
}
```

where "invite" could alternatively be "open".

Adds API to set the 'joinable' flag, and corresponding flag in the
table.

404 correctly on missing paths via NoResource

fixes https://github.com/matrix-org/synapse/issues/2043 and https://github.com/matrix-org/synapse/issues/2029

Use .iter* to avoid copies in StateHandler

Fix replication after switch to simplejson

rearrange ordering of releases to match chronology

Turns out that simplejson serialises namedtuple's as dictionaries rather
than tuples by default.

bump version

Fix bug where state cache used lots of memory

The state cache bases its size on the sum of the size of entries. The
size of the entry is calculated once on insertion, so it is important
that the size of entries does not change.

The DictionaryCache modified the entries size, which caused the state
cache to incorrectly think it was smaller than it actually was.

CONTRIBUTING.rst: fix CI info

Update purge_history_api.rst

Doc: Make the event_creator routes regex a code block

clarify that `purge_history` will not purge state