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.

Fixes a bug introduced in https://github.com/matrix-org/synapse/pull/1783 which
meant that single backslashes were not allowed in event field filters.

The intention here is to allow single-backslashes, but disallow
double-backslashes.

Revert "support admin_email config and pass through into blocking errors, return AuthError in all cases"

This reverts commit 0d43f991.

 * use a valid filter in rest/client/v2_alpha test

Signed-off-by: pik <alexander.maznev@gmail.com>

Signed-off-by: pik <alexander.maznev@gmail.com>

 * add invalid filter tests

Signed-off-by: pik <alexander.maznev@gmail.com>

Make sure that a user cannot pretend to be a guest by adding 'guest = True'
caveats.

Record the device_id when we add a client ip; it's somewhat redundant as we
could get it via the access_token, but it will make querying rather easier.

 - Add unittests for client, api and handler

Signed-off-by: Negar Fazeli <negar.fazeli@ericsson.com>

Also check that the __repr__ method for FilterCollection does something
sensible.

This tracks data about the entity which made the request. This is
instead of passing around a tuple, which requires call-site
modifications every time a new piece of optional context is passed
around.

I tried to introduce a User object. I gave up.