Use emma to learn where our bridge distribution mechanisms (don't) work

We have BridgeDB usage metrics that allow us to infer where our HTTPS frontend works and probably doesn't work. This is not as easy for email and for moat because we currently don't see the source address of a client (see legacy/trac#32276 (moved)).

Emma however can tell us if a user can use each of BridgeDB's distribution mechanisms:

1. It checks if the page behind https://bridges.torproject.org contains the string "The Tor Project" (for the HTTPS distributor).
2. It checks if the page behind https://ajax.aspnetcdn.com contains the string "Microsoft Ajax Content Delivery Network" (for the moat distributor).
3. It checks if the page behind https://accounts.google.com/ServiceLogin contains the string "Sign in" (for the email distributor).
4. It checks if the page behind https://mail.riseup.net/rc/ contains the string "Welcome to mail.riseup.net" (also for the email distributor).

What remains is to ask volunteers in different countries to run emma, so we can get a better idea of where our distribution mechanisms (don't) work.