Synchronize user's Discord roles with their GitHub roles on account link

[jchristgit]

Follow-up to #357: when a user links accounts together, give them the roles that
they deserve on GitHub.

[jchristgit]

Blocked by python-discord/infra#498.

[jb3]

No longer blocked on python-discord/infra#498.

A new mailusers group has been implemented which Dovecot SASL checks for membership of, only groups who previously had access to mail continue to have this access, the newly introduced helpers groups will not get this (though they will still have forwarded mail support).