Server: Make auth and discussion location optional.

This allows running the server in "passive" mode, sending only
emails, not posting to Github.
Server: Return 403 on bad Github secret instead of 500
Relax text dependency upper bound to 1.3
Pass --from to git send-email.

Older gits (e.g. Debian 7) keep asking
  "Who should the emails appear to be from?"
even when --confirm=never is given. This stops them from asking.
Server: Improve CLI error handling.

More descriptive error messages.
Don't use `clone --depth 1` for base repo.

This can lead to the whole repo history being sent as patches.
Merge pull request #14 from nh2/wip

Various minor improvements. Most of them UI, and one to prevent mailing the whole repository history.