Replace API collection paths with search parameters

[obulat]

Problem

Additional search views in the API should use the /search endpoint with query parameters instead of separate endpoints with path parameters.

Description

The following changes are necessary:

• remove the new endpoints (tag, creator and source)
• update the serializer to use unstable__collection and also add unstable_tag (singular, in addition to plural tags that's used for search within tags instead of exact match)
• update the build_collection_query method

Additional changes

During the writing and review process, the following changes were also added:

• the MediaSearchRequestSourceSerializer was merged into the MediaSearchRequestSerializer. We now need to add media_type to the serializer context when calling it.
• SHOW_COLLECTION_DOCS env variable allows to show the updated documentation for the search endpoint. When it's set to False, no changes are shown in the API doc site.

Additional context

The changes were discussed in Rethinking additional search views URLs