There are a number of instances where a server or admin may puppet a
user to join/leave rooms, which we don't want to fail if the user has
not consented to the privacy policy. We fix this by adding a check to
test if the requester has an associated access_token, which is used as a
proxy to answer the question of whether the action is being done on
behalf of a real request from the user.

Setting this to 50 or so makes a bunch of sytests fail in worker mode.

Rate-limit outgoing read-receipts as per #4730.

Add note on tuning postgres

Add py27-old test case to buildkite

Use a regular HomeServerConfig object for unit tests

Enforce hs_disabled_message correctly

turns out this relies on there being a `user_consent_version` set.

Rather than using a Mock for the homeserver config, use a genuine
HomeServerConfig object. This makes for a more realistic test, and means that
we don't have to keep remembering to add things to the mock config every time
we add a new config setting.

Make sure that we have a `server_notices_mxid` set, given that we are relying
on it.

* Set allow_guest_access = True, since we rely on it
* config doesn't have a `hostname` attribute; it is `server_name`

The Mailer expects the config object to have `email_smtp_pass` and
`email_riot_base_url` attributes (and it won't by default, because the default
config impl doesn't set any of the attributes unless email_enable_notifs is
set).

Fixes a bug where hs_disabled_message was not enforced for 3pid-based requests
if there was no server_notices_mxid configured.