Here are the overall changes I'm adding in this PR:

- Don't create a new thread when switching to a new external agent while
in parked draft state
- Include parked draft threads in the thread switcher
- Add the draft thread item in the sidebar when navigating away from a
draft, as opposed to only on cmd-n/new thread
- Upon deleting content from a parked draft, remove its entry from the
sidebar. From this point on, we convert that draft back into the
ephemeral state so it behaves the same way as a fresh new draft would.

Release Notes:

- N/A