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

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
Does this work as expected? I thought we had a problem where it was only filtering things that had all these properties, instead of any of these properties. If you tested it, then no worries. I was expecting something like `.exclude(slug='').exclude(current_revision=None).exclude(...)`