[WIP] Sharelink 766394

This addresses bug 766394, which is linked at the bottom of this comment. 
Currently done:
* KB share links generated on submission of /kb/new
* Schema migrations for a booleanfield to show the share link or not and the share link itself

To do:
* Data migration for existing KB articles
* Tests for the generate_short_link function

https://bugzilla.mozilla.org/show_bug.cgi?id=766394
generally when there are more args than fit on a line, we will put one per line, like this:

```python
endpoint = reverse('wiki.document',
                   locale=doc.locale,
                   args=[doc.slug])
```

this makes things easier to read.