Remove unnecessary UsernameCipher code

UsernameCipher was copied from edx-platform, but it is not necessary
because the encrypted token needed for the unsubscribe URL is contained
in the user's preference that is also included in the payload from the
LMS user API.
Merge pull request #24 from edx/gprice/remove-username-cipher

Remove unnecessary UsernameCipher code
TNL-301 Filter posts by group_id.
fixup: updates based on review comments.
fixup: remove unneeded changes in digest.py and test_digest.py.
fixup: review comments.
fixup: review comments.
fixup: review comments.
fixup: review comments.
fixup! TNL-301 Filter posts by group_id.

Fix management commands
WIP - setdefault()s removed, tests broken
fixup: fix unit tests.
Fix management commands more
don’t send courses without any threads, or digests without any courses
don’t use a special test runner