feat: add relevance sort to sorting options

[edward-ly]

Closes #1298, replacing rating as the default sort with and without a search query. At the moment, it's only partially tested; I may refine results a bit more depending on the various search results I get.

[bigcat88]

http://127.0.0.1:8000/?order_by=relevance&ordering=desc&search=app_api
4.2.14
NotSupportedError
Calling QuerySet.prefetch_related() after union() is not supported.
/Users/user/PycharmProjects/appstore/.venv/lib/python3.11/site-packages/django/db/models/query.py, line 1960, in _not_support_combined_queries
nextcloudappstore.core.views.CategoryAppListView
/Users/user/PycharmProjects/appstore/.venv/bin/python

[edward-ly]

I finally figured out a workaround to the union operator, so the sort should be working now.

[bigcat88]

seems to work without errors - everything is great

[github-actions]

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