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.

Make it so that most options in the config are optional, and commented out in
the generated config.

The reasons this is a good thing are as follows:

* If we decide that we should change the default for an option, we can do so,
  and only those admins that have deliberately chosen to override that option
  will be stuck on the old setting.

* It moves us towards a point where we can get rid of the super-surprising
  feature of synapse where the default settings for the config come from the
  generated yaml.

* It makes setting up a test config for unit testing an order of magnitude
  easier (see forthcoming PR).

* It makes the generated config more consistent, and hopefully easier for users
  to understand.

Add two ratelimiters on login (per-IP address and per-userID).