Fix a typo in the extension docs.

In the docs dealing with extension configuration pages, one of the citations
said "admin_url.py" where it should have said "admin_urls.py". Trivial fix.
Fix the fill-database command.

Changes in the diff code as well as django-1.5 caused fill-database to break.
This change fixes it, switching to use the UploadDiffForm from the reviews
module, and calling is_valid() to make sure that the form cleans the input
data.

Testing done:
Ran the command and saw that it actually ran and populated the database
correctly. Ran the unit tests, they all pass.

Reviewed at https://reviews.reviewboard.org/r/4692/
Add a sample GitHub token setting to settings_local.py.tmpl.

This adds commented out, documented GITHUB_CLIENT_ID and
GITHUB_CLIENT_SECRET settings to the settings_local.py.tmpl, which is
generated when doing a prepare-dev.py.

Reviewed at https://reviews.reviewboard.org/r/4665/