Solves an issue with using git > 1.8.1 for github based pip requirements.

Two endpoints are intended to be used by AJAX calls from the web UI with an
authenticated user. When enabled, the value of the preference is an encryption
of the username, to be used as a token for one-click unsubscribe links. The
third endpoint is the target of unsubscribe links, which displays a page with
an appropriate message to the user.

The new API uses Django REST Framework. For now, it is designed specifically
to support the use cases required by the forum digest notifier (not yet built),
with a goal of making it more generally useful over time.

Update existing tests and provide new ones to test new paradigm.

Refactor new set_many and update XBlock version number.

Replacing the get_request_method function everywhere

in which missing lines were over-reported

to `origin/master` instead of `master`)

There is no package called `mysql` in Debian/Ubuntu - instead, MySQL is
split into `mysql-client` and `mysql-server`.

Also deeted duplicate package name.