Revise Draftail integration UI

[tomusher]

Right now the Wagtail AI is a bit rough, some thoughts for potential refinements:

• In place of a dropdown when you click the 'magic wand' button, perhaps a modal makes more sense? This would give us more space to describe each prompt.
• Move the loading indicator - it currently blocks the editor. If we went the modal route, this could be contained within the modal.

[Morsey187]

Following up on my disscusion with Ben E, he's keen to do some usability testing as part of the wagtail design backlog (see ticket) before making suggestions on larger changes here (i.e. use of modals), as the design considerations ties into existing UI discussions on features such as content quality checkers and we would also need some general usability testing around the questions raised in that ticket to know what's best.

However, we also discussed minor design improvements we could make which could be done as part of this ticket which include:

• Updating the dropdown to use existing stlying from the nav dropdown
  dropdown reference

• Updating the error message to something more generic on failed requests
• Updating background colour on the loading indicator for dark theme (white text is currently hard to read)
• A "cancel request" button under the loading indicator text using wagtail button styling
  Button reference