`ActiveThread` and `MessageEditor` only make sense when `active_view` is
`Thread`, so we moved them in there. This will make it easier to work on
new agent threads.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>