Integrate GetTor into rdsys

Both GetTor and BridgeDB seem have a lot of requirements in common when it comes to email processing and configuration. (For instance, mail filtering, DKIM checks and so on)

Maybe we should sync the email processing code into something like a python tor email library for both at some point in the future.

Since we are reconfiguring BridgeDB to use rdsys as a backend, let's also make sure rdsys is generalizable enough to handle distributing GetTor links.