Allow longer & shorter usernames, complying with the MXID length spec

[sandhose]

This adds the server_name to the policy data, which lets us compute the MXID, which means we can accurately make sure we allow MXIDs up to 255 characters long.
It also makes the minimal username length 1 instead of 3.

I also made it so that we get 'error codes' from the policy violations, which lets us translate more specific error messages

Fixes #2454
Fixes #2620

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[reivilibre]

are these policies overridable by the admin? It doesn't seem sensible to let an admin override the maximum length (or alphabet check, particularly if it might allow unicode user IDs).
I would say to be careful that count counts bytes and not 'characters', but if we restrict to the restricted set of ASCII then that's fine.

[sandhose]

are these policies overridable by the admin? It doesn't seem sensible to let an admin override the maximum length (or alphabet check, particularly if it might allow unicode user IDs).

It's anyway calling Synapse's 'username available' endpoint, which checks that as well. This is mainly making the error better for the end user