Modify the backend notification script to handle saved searches

[chouinar]

Summary

Logic should be roughly:

• Fetch all saved searches since the last job ran (TODO - how to know when the last job ran?) based on the last_notified_at timestamp
• Merge all saved searches with the same query together (maintain which records in the DB these connect to)
• Query the search index with all the searches, only fetching opportunity IDs
• For any searches that produce a different set of results, update the user notification mapping + user_saved_search table.

TODO - fill out in a bit more detail

The job does not need to actually notify, it can just write to the table created in #3546

Acceptance criteria

• Backend script updated