gettor should automatically include bridges in its answer

If somebody gettors from their gmail address, we should go to bridgedb and fetch the bridges they would have gotten, and include them in the text we mail back.

To finish this ticket we'll need to reenable dkim checking on gettor mails (which is part of legacy/trac#3381 (moved)), but we'll also need to come up with some sort of secure way to ask bridgedb for its answer.

I expect that second step to be messy, so marking this ticket 'minor' priority.