Make conversations optionally shareable

[sabaimran]

Incoming

• Shared conversations are viewable by anyone, without a login wall
• Can share a conversation from the three dot menu
• Add a new model for Public Conversation
• The rationale for a separate model is that public and private conversations have different assumptions. Separating them reduces some of the code specificity on our server-side code and allows us for easier interpretation and stricter security. Separating the data model makes it harder to accidentally view something that was meant to be private
• Add a new, read-only view for public conversations. It has a list of available agents on the side panel.

[debanjum]

This is great! It'll simplify sharing responses by Khoj with others instead of the previous clunky copy/paste each message + mention it was generated by Khoj mechanism