Correctly wire in update group profile over federation

PEP8 fixes

E741 says "do not use variables named ‘l’, ‘O’, or ‘I’".

what could possibly go wrong

Fix a logcontext leak in the media repo

Updates to ID checking

Remove pointless create() method

May as well do it whenever we parse a Group ID. We check the sigil and basic
structure here so it makes sense to check the grammar in the same place.

... because the spec says we should.

Factor out a common function for checking user ids and group ids, which forbids
capitals.

It just calls the constructor, so we may as well kill it rather than having
random codepaths.

Add config to enable group creation

Enforce sensible group IDs

Fix typo in thumbnail generation

Fix 500 error when we get an error handling a PDU

Fix 500 error when fields missing from power_levels event

FederationServer doesn't have a send_failure (and nor does its subclass,
ReplicationLayer), so this was failing.

I'm not really sure what the idea behind send_failure is, given (a) we don't do
anything at the other end with it except log it, and (b) we also send back the
failure via the transaction response. I suspect there's a whole lot of dead
code around it, but for now I'm just removing the broken bit.

If the users or events keys were missing from a power_levels event, then
we would throw 500s when trying to auth them.

fix thumbnailing (#2548)

`os.path.exists` doesn't allow us to distinguish between permissions errors and
the path actually not existing, which repeatedly confuses people. It also means
that we try to overwrite existing key files, which is super-confusing. (cf
issues #2455, #2379). Use os.stat instead.

Also, don't recomemnd the the use of --generate-config, which screws everything
up if you're using debian (cf #2455).