[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
Nit: Using '\' to escape quotes in strings is kind of gross, if you can avoid it. I suggest doing something like `u"If checked, show the 'Share this link' text."`. i.e. just use double quotes on the outside. Alternatively, it might be good to use double quotes inside (like `u'If checked, show the "Share this link" text.'`)