[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
You can probably be smarter about this and automatically figure this out based on category, is_archived, etc.