Implement session branching

[stippi]

• The user can edit past messages (all user messages have an edit button)
• This loads the past user message into the input area for editing. The conversation is cleared of any subsequent messages including the edited user message.
• Submitting the message creates a branch in the session. The user message now has more than one version, and little branch switching controls allow to switch the active branch.

Caveats:

• GPUI only for now
• Nothing is done to the source code. If you need to revert to an older state of the code for the branch to make sense, you need to do so manually.