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.

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

This reverts commit 0d43f991.

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>

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.

This follows the same flows-based flow as regular registration, but as
the only implemented flow has no requirements, it auto-succeeds. In the
future, other flows (e.g. captcha) may be required, so clients should
treat this like the regular registration flow choices.

A couple of weird caveats:
 * If we can't validate your macaroon, we fall back to checking that
   your access token is in the DB, and ignoring the failure
 * Even if we can validate your macaroon, we still have to hit the DB to
   get the access token ID, which we pretend is a device ID all over the
   codebase.

This mostly adds the interesting code, and points out the two pieces we
need to delete (and necessary conditions) in order to fix the above
caveats.

Removes device_id and ClientInfo

device_id is never actually written, and the matrix.org DB has no
non-null entries for it. Right now, it's just cluttering up code.

This doesn't remove the columns from the database, because that's
fiddly.

We're about to have two kinds of token, access and refresh

Add logic to map the appservice token to the autogenned appservice user ID.
Add unit tests for all forms of get_user_by_req (user/appservice,
valid/bad/missing tokens)