[bug 766394] Moved short_link generation code.

It is now done with a celery task because it's an API call and we
want it to be asynchronous. Additionally, I added more descriptive
exceptions for any status code responses form bit.ly's api other
than a 200.
[bug 766394] Fixed util reference mistake in forms
[bug 766394] Changed quit conditions.

We now quit the generate_short_url function if either a username
or API key is not found. There was also an update to an error
message.
[bug 766394] Cron job for generating share links.

There is now a cron job for generating share links where they
are missing.
[bug 766394] Updated settings to match tasks.

We need to make sure the bitly settings are None by default, so
that if the settings are missing it is noticed by
generate_short_link in wiki/tasks.py