* It's supposed to be purge_local_events, not ..._history
* Fix the doc to have valid json

Allow use of higher versions of saml2

Tell storage providers about new file so they can upload

delete_local_events for purge_room_history

... to avoid locking the table for too long

Add a flag which makes the purger delete local events

it was a bit of a non-sequitur there

this thing takes ages and the only sign of any progress is the logs, so having
some logs is useful.

(beacause it deletes more than state)

Remove pointless ratelimit check

The intention was for the check to be called as early as possible in the
request, but actually was called just before the main ratelimit check,
so was fairly pointless.

 Split event creation into a separate handler

* Split state group persist into seperate storage func

* Add per database engine code for state group id gen

* Move store_state_group to StateReadStore

This allows other workers to use it, and so resolve state.

* Hook up store_state_group

* Fix tests

* Rename _store_mult_state_groups_txn

* Rename StateGroupReadStore

* Remove redundant _have_persisted_state_group_txn

* Update comments

* Comment compute_event_context

* Set start val for state_group_id_seq

... otherwise we try to recreate old state groups

* Update comments

* Don't store state for outliers

* Update comment

* Update docstring as state groups are ints

Remove redundant return value from _calculate_state_delta

Factor out common code for search insert

we already have the state from _get_new_state_after_events, so returning it
from _calculate_state_delta is just confusing.

we can reuse the same code as is used for event insert, for doing the
background index population.

... as a precursor to making event storing and doing the bg update share some
code.

montoring metrics for number of cache evictions

Ensure media is in local cache before thumbnailing

Handle url_previews with no content-type

Factor out resolve_state_groups to a separate handler

factor _get_new_state_after_events out of _calculate_state_delta

Docstring fixes

Add an admin route to get all the media in a room

avoid failing with an exception if the remote server doesn't give us a
Content-Type header.

Also, clean up the exception handling a bit.

Fix a couple of errors in docstrings

The return type was a complete lie, so fix it

We extract the storage-independent bits of the state group resolution out to a
separate functiom, and stick it in a new handler, in preparation for its use
from the storage layer.