experimental: allow user to give instruction for the conversation

[louis-jan]

Problem

• Chat body seems boring after created, blank state is not decorated well
• As a user I want to give the conversation context (set system prompt)

Solution

• Before the Assistant settings feature is designed, this could be an experimental feature so users can get how it is working (also to test against models / endpoints)

TODO:

• Its better to have a toggle for every single dark feature

[louis-jan]

@urmauur, this is the initial feature implementation. Please help to update it with a better UX