Merge pull request #550 from matrix-org/daniel/guestnames

Allocate guest user IDs numericcally

The current random IDs are ugly and confusing when presented in UIs.
This makes them prettier and easier to read.

Also, disable non-automated registration of numeric IDs so that we don't
need to worry so much about people carving out our automated address
space and us needing to keep retrying ID registration.