Moves all things related to creating the URLs of the mod platforms
that go to network tasks to a single place, so that:
1. Maintaining and fixing eventual issues is more straightforward.
2. Makes it possible to factor out more common code between the
different modplatform pages
This creates a hierarchy in which ModPage and ModModel are the parents
of every mod provider, providing the basic functionality common to all
of them.
It also imposes a unique .ui file (they were already equal before, just
duplicated basically) on all mod providers.
