Add loading indicator to overlay drawers

[beepdotgov]

Surfacing some discussion from #483 as a new issue. Namely:

Right now, the drawers are opening beautifully, but there’s a second step before the user can interact with them: a drawer slides into place with a “loading form” message, and then the content appears:

Screen.Recording.2024-09-11.at.1.50.47.PM.mov

Is it possible to skip that middle step, and open the drawer once the form’s properly loaded?

Per @echappen, it would quite a bit of reworking of the React component hierarchy in order to accomplish this, which is why we’re opening up this ticket.

Can this task be done in one increment?

TBD. Labeling as “research” so we can assess scope.

Acceptance Criteria

• There is an approved design for a loading animation, which fires before an overlay drawer appears.
• The design has been implemented in the Dashboard prototype.