[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
this should be *share* links, not short links.