Allow for non-web-based consumers

Some consumers are not web sites, so they don't really have a sane value
to enter for the callback URL.

This change allows Special:MWOAuth/verified to be used as the callback
URL, which will display the verification code and instructions to copy
this code into the consumer application.

Change-Id: I0193101050419c42f981c61c440880a4cda1fdf1
Merge "Allow for non-web-based consumers"