[bug 766394] Added migration for  migrations user.

This is the user we should use for data migrations when having
to assign owners/creators to new objects.
[bug 766394] Auto-generation for share links.

Share links for KB articles should now automatically generate
if the articles are in the correct categories. A cron job runs
every day at midnight as well to add share links where there should
have been some, but currently aren't.

Additionally, the bitly api key and username need to be entered in
settings.py for share links to generate.