Add experimental chat database entities

[pileks]

These are not meant to be applied to production, but rather as a source of entities for Steven to try out a Chat UI on.

None of the entities touch on anything existing in the database, so it's safe to set up and remove later.

Notes:

• Have a separate database to play around with? With its own indexer?
  • Or a separate experimental schema?
  • Maybe even a fully local setup? (Could be a nice piece od devexp, but will require effort)