pulling the avatars down from the "people" partial, and pulling the
add button up from the "assignment" partial.
The result is:
- the "bubbles/sidebar/assignment" partial is just the dialog box
- the "bubbles/assignees" partial contains everything about assignees
- the "bubbles/people" partial makes one render call
and I've introduced some turbo frames in the expectation of the next
commit which will load the dialog in the background.