Add capability to create combined summary of several uploaded documents

[rockett-m]

Check select box on documents uploaded in the left panel for which to summarize. This feature has been requested by several people.

Imagine a few chapters of a textbook, academic articles, industry news, etc., that you need to understand quickly.
A summary that ingests all the information and produces a combined summary would save a lot of manual reading time.

Speedup hint: if a doc has already been summarized, the summary should be stored in RDS or Postgres, so retrieve those and summarize any new docs uploaded (save those) and add a new function+prompt to produce multi source summary. Will be important to reference the authors and titles of each source. Otherwise it could be very generic like "all these sources talk about AI ..."