Add email support.

[ponychicken]

Users should be able to leave their email, so that they get a followup if there are questions.

[emka]

depends on #49

[emka]

If a non-registered user can subscribe an email address to a bug this could be abused to send spam. I prefer a solution where registered users can "watch" a bug, which will be possible at least for the bugs youa user created in the "notes integration", see #49