Add UI for AI Chat

[DavidMStraub]

This adds the UI for the proposed chat feature in Web API, see https://github.com/gramps-project/gramps-web-api/pull/539.

What's missing?

• [x] A button to clear the search history/start a new topic
• [x] UI for managing semantic search index
• [x] Possibly, exposing semantic search in the search view
• [x] UI for managing permissions, once implemented
• [x] UI for viewing quota, once/if implemented

[DavidMStraub]

Docs: https://github.com/gramps-project/gramps-web-docs/pull/18