Add backend topic search
The purpose of the code is to be able to search for topics, and ranking them by their popularity in the last N days. This requires adding topic string and date posted indexing and fields to opensearch with special types for fast aggregation, and (maybe) a new endpoint that does this new aggregation query.
This gives us a good topic search as well as a free trending topic list for free if you don't provide a search query
Missing:
- "Migration" to add topic field to opensearch documents for existing events
- Tests