Offline SUMO Kitsune API

I'm not quite done for the server side API so please do not merge this just yet. It is here for feedback as I think it is fairly stable at this point in time. There are some detailed documentations at docs/offline.rst

Before this can be merged, the following tasks have to be completed:

 - [x] **Write unittests!**
 - [x] Complete documentations
 - [x] Get rid of the `get-language` API
 - [x] [Dropped] Limit CORS API to a certain domain only
 - [x] Move API into a nightly batch job (Maybe)
 - [x] [Dropped] Write API for downloading diffs
 - [x] In addition to this, I would like to start migrating [osumo](https://github.com/shuhaowu/osumo) under the mozilla organization if possible.

It is also possible to try all of this yourself by cloning the osumo repository and running the server there.