Limit in flight DNS requests

Follow-up to #5124

Also added a bunch of checks to make sure everything (both the stuff added on #5124 and this PR) works as intended.

Ratelimit 3pid invites

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.

... after it got broken in 1565ebec.

Synapse 0.99.3.2 (2019-05-03)
=============================

Internal Changes
----------------

- Ensure that we have `urllib3` <1.25, to resolve incompatibility with `requests`. ([\#5135](https://github.com/matrix-org/synapse/issues/5135))

Build fixes

psycopg 2.8 is now out, which means that the C library gets built from source,
so we now need libpq-dev when building.

Turns out the need for this package is already documented in
docs/postgres.rst.

Blacklist 0.0.0.0 and :: by default for URL previews

Use SystemRandom for token generation.

Add some limitations to alias creation

Rather than copying-and-pasting the same four lines hundreds of times

Move the admin API to `/_synapse/admin/v1`

It doesn't really belong under rest/client/v1 any more.

Fix path in account validity admin route's doc

* Add a default .m.rule.tombstone push rule

In support of MSC1930: https://github.com/matrix-org/matrix-doc/pull/1930

* changelog

* Appease the changelog linter