Added ChromaDB vector storage support

optionally search one language only

option to prefer a configured language instead of guessing

use a cut-off point when considering similar documents

refactoring to make models selectable

This makes it much easier to add new models. Models can now be selected
via the configuration

Use overlapping chunks, prepare for new models

allow to restrict access to certain users and groups

add optional conversation logging

This might be useful to debug and monitor the real world usage of the
bot and analyze abuse.

german translation and more styling adjustments

Check ACLs before using sources

support chatting

asking standalone question works

initial checkin. working vector storage and similarity search