Redirect renamed project slugs in API v2 project endpoint (fixes #4182)

[eshu3104]

/api/v2/projects/<old-slug>/ returned a 404 for renamed projects, while the
web UI redirects to the new slug. This adds a retrieve() override to
ProjectIndividualView with the same ProjectSlugHistory fallback: on a
failed lookup, respond with a 302 to the current slug's URL. Unknown slugs
still return 404.

The fallback logic mirrors get_project_or_redirect() in
pontoon/base/services.py. The 404 path now runs one extra history query,
hence the query-count update in test_disabled_project.

Tests: test_project_renamed_slug_redirects (old slug returns a 302 with the
correct Location) and test_project_unknown_slug_404s.

Note: other slug-based endpoints and locale endpoints (via LocaleCodeHistory)
have the same gap. This PR only covers the endpoint from the issue; happy to
follow up on the rest.

[functionzz]

Looks good to me, if possible, can you extend the behavior to the /api/v2/locales/<locale_code> & /api/v2/<locale_code>/<project_slug> endpoints? The LocaleCodeHistory & ProjectSlugHistory models will be useful :).

You will need to edit the behavior of LocaleIndividualView & ProjectLocaleIndividualView in a similar manner as you have done here.