Add a plugin progress indicator

[ruthhammond]

What

Add an indicator to the plugin UI to show the progress of installing/uninstalling plugins

Why

So that users know that something's happening, particularly if there's a slow connection. This user experience will match what happens for installing/uninstalling templates

Who needs to work on this

Designer Developer

Who needs to review this

Designer Developer

Done when

• [ ] There is a design for the progress indicator
• [ ] The design is implemented

[BenSurgisonGDS]

See the PR Add Plugin progress indicator for a possible implementation