origin_save: Filter out visit type with no scheduler load-* task type

In order to ease the integration of new visit types for the "Save code now"
service, filter out those with no associated task type in scheduler db.

It means that a new visit type can be added to "Save code now" implementation
but it will not be available until the associated scheduler load task type
gets created. It enables to test the new visit type in dev environments
and deploy safely to production if the associated scheduler load task type
did not get created yet.

Also remove the internal endpoint to get savable visit types, those can
be simply added in Django template.

As that change broke a lot of tests due to scheduler being previously
mocked, most of the tests related to "Save code now" got improved by
removing the mocks and using the swh_scheduler pytest fixture.

Related to D5992