Add docstring for email.server.MailMessage.getClientAddress.
Add unittests for bridgedb.email.templates module.
Add a personalised greeting and footer to emails.

 * FIXES a problem noted by Robert Ransom (in ticket #5463) with
   adversaries being able to capture a signed email for Alice and resend
   it to Bob to target Bob by having influence over his available
   bridges.

 * ADD a footer to emails, which includes the client's email address and
   a timestamp for when the email was generated.

 * ADD the answer bridge lines to the answer template creation
   parameters, so that the answer string isn't formatted into the
   message after message creation.

Rename email.template functions for adding text. To distinguish them
from the templates.build* functions, which build entire emails. The
following functions have been renamed:

 * RENAME templates.buildCommands()     → templates.addCommands()
 * RENAME templates.buildKeyfile()      → templates.addKeyfile()
 * RENAME templates.buildHowto()        → templates.addHowto()
 * RENAME templates.buildBridgeAnswer() → templates.buildBridgeAnswer()
Remove password from gnupghome/TESTING.subkeys.sec key.
Merge branch 'fix/5463-sign-client-email-addr' into develop