cgit: rewrite the CGit lister

Simplify the code:

• do only inherit from ListerBase
• implement HTTP queries directly using requests
• get rid of convoluted code

Make the origin_url gathered from the git repo's "project" page instead of
building it from the 'url_prefix' hack. Now, the lister WILL make substancially
more requests, since it will make one request per listed git repo, but
the provided origin_url should be pretty reliable now.

When several url are provided as clonable URLs, choose the http/https one first,
otherwise, choose the first one of the list.

Add proper tests for the cgit lister.

Also, get rid of the 'time_updated' column in the model.