Feature: Combined book+audiobook downloads (#773)

- Adds a combined search option in the search bar selector
- Choose both a book and audiobook file in a two-step release modal, and
download both simultaneously from a single search result.
- Works for requests. Request both a book+audiobook at once, or works
seamlessly with request policies that differ between book + audiobook
(E.g. automatically download the ebook portion, while the audiobook gets
sent as a request)
- Hidden for users who have book or audiobooks blocked. 

Closes #611

tests/core/test_admin_users_api.py

@@ -1202,8 +1202,10 @@ class TestAdminSearchPreferences:
         assert data["tab"] == "search_mode"
         assert data["keys"] == [
             "SEARCH_MODE",
+            "SHOW_COMBINED_SELECTOR",
             "METADATA_PROVIDER",
             "METADATA_PROVIDER_AUDIOBOOK",
+            "METADATA_PROVIDER_COMBINED",
             "DEFAULT_RELEASE_SOURCE",
             "DEFAULT_RELEASE_SOURCE_AUDIOBOOK",
         ]