fix: /rss lazy load issue & perf improvements

Some fixes for the /rss endpoints

• Load all data in one go:
  Previously data was lazy loaded thus it made several sub-queries per package (> 200 queries for composing the rss data for a single request).
  Now we are performing a single SQL query (request time improvement: 550ms -> 130ms).
  This also fixes aurweb-errors#510 and alike

• Remove some "dead code":
  The fields source, author, link were never included in the rss output (wrong or insufficient data passed to the different entry.xyz functions).
  Nobody seems to be missing them anyways, so let's remove em.

• Remove Last-Modified header:
  Obsolete since nginx can/will only handle If-Modified-Since requests for static files in it's current configuration.
  All requests are passed to fastapi anyways.