save_code_now: Move feature into a dedicated django app

Move all templates, views, urls and commands related to the save code now
feature into its dedicated django application.

Also introduce a new django command rename_app to easily rename a django
application when developing or deploying.

Previously the save code now models were located in the swh.web.common
application that will be removed so that is why that application renaming
to swh.web.save_code_now is mandatory to avoid errors when running migrations.

Related to T4398