Fix #1677 (dynamic thumbnails aspect)

Strip newlines from SQL queries

Fix a race in transaction queue

It was theoretically possible for a PDU to get queued and not sent for ages. On
closer inspection I think there were bigger problems elsewhere, but we might as
well fix this since it's easy.

Fix /context/ visibiltiy rules

Add an example log_config file

add trailing NL

Make the pushers lang field column longer

Allow forgetting rooms you're banned from

While we're changing anyway

To accommodate things like zh-Hans-CN

Fixes https://github.com/vector-im/riot-ios/issues/1031

Store the default push actions in a more efficient manner

Aggregate event push actions

Fix bugs in the /keys/changes api

Make kick & ban reasons work

We somehow specced APIs with reason strings, preserve the content
in the events  and even have the clients display them, but failed
to actually pass the parameter through to the event content.

* `get_forward_extremeties_for_room` takes a numeric `stream_ordering`. We were
  passing a `RoomStreamToken`, which meant that it returned the *current*
  extremities, rather than those corresponding to the `from_token`. However:
* `get_state_ids_for_events` required a second ('types') parameter; this meant
  that a `TypeError` was thrown and we ended up acting as though there was *no*
  prev state.
* `get_state_ids_for_events` actually returns a map from event_id to state
  dictionary - just looking up the state keys in it again meant that we acted
  as though there was no prev state. We now check if each member's state has
  changed since *any* of the extremities.

Also add/fix some comments.