Admin API: Set adminship of a user

This endpoint isn't much use for its intended purpose if you first need to get
yourself an admin's auth token.

I've restricted it to the `/_synapse/admin` path to make it a bit easier to
lock down for those concerned about exposing this information. I don't imagine
anyone is using it in anger currently.

Signed-off-by: Joseph Weston <joseph@weston.cloud>

Allow for the creation of a support user.

A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.

People keep asking why their database hasn't gotten smaller after using this API.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Presumably this is the intention anyways. I've also updated the domain part to be something more along the lines of what people might expect.

clarify that `purge_history` will not purge state

Make the purge request return quickly, and allow scripts to poll for updates.

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

Add a flag which makes the purger delete local events

Signed-off-by: Travis Ralston <travpc@gmail.com>

make it absolutely clear that Purge History API does not remove all traces of events and message contents

because this topic pops up too often

#890 #1621 #1730 #2260 #2315 and so on

I haven't (yet) documented all of the user-list APIs introduced in
https://github.com/matrix-org/synapse/pull/1784 because the API shape seems
very odd, given the functionality.

purge_media_cache takes its arg from a query-param, not the POST body, for some
reason.