fix(admin): keep base_catalog pre-filled after save and add another

[ccantillo]

sticky base_catalog on "Save and add another"

Keep the Base catalog field pre-filled when an admin clicks
"Save and add another" in the BaseCatalogCourse admin page.

Context

Related issue: sticky base_catalog after save-and-add-another in
/admin/partner_catalog/basecatalog/.

When adding multiple courses to the same BaseCatalog one by one,
clicking "Save and add another" redirected to a blank add form,
forcing the user to re-select the catalog every time. The only
workarounds were going back to the previous page or copy-pasting
the catalog ID manually.

Root cause

Django admin's default response_add redirect to /add/ does not
carry any query parameters, so the base_catalog raw-id field is
always empty on the next form load.

Change

Two small overrides added to BaseCatalogCourseAdmin in
partner_catalog/admin.py:

• response_add — when _addanother is in the POST data,
  redirects to /add/?base_catalog=<id> instead of the plain /add/,
  preserving the catalog that was just used.
• get_changeform_initial_data — reads the base_catalog GET
  parameter and injects it into the form's initial data so the
  raw-id widget renders with the value already set.