Pagable and efficient search

[ssddanbrown]

This PR aims to change how searches are done, to use a single core query instead of a query per entity type.
This should make the search more efficient, while resulting in a stable & consistent response item count.

Todo

• Debug large response processing time gap between making the queries, and rendering the views.
• Update API search endpoint description.
• Address remaining TODOs
• Test other search controller methods (anything which uses SearchRunner).
• Test edge-case scenarios (different search types, filtered entity types, no tags in results, no parent items etc...)
• Testing