[Bug 766394] Auto-generate share links for KB articles

This addresses bug 766394, which is linked at the bottom of this comment. 

* [x] KB share links generated on submission of /kb/new
* [x] Schema migrations for a booleanfield to show the share link or not and the share link itself
* [x] Data migration for existing KB articles
* [x] Tests for the generate_short_link function

https://bugzilla.mozilla.org/show_bug.cgi?id=766394