Implement new install button flow in details page

See the individual commits for descriptions of changes. This implements the new design here.

The draft status is just to remind me to drop the WIP testing commit before merging. Other than that, this is ready to merge (pending review).

Differences from the design / things to note:

• Instead of showing the download speed (“4.8 MB/s”) the percentage progress is shown, as this is what the code has access to (exposing the download speed would be significant work)
• Not shown in the design, but implemented: the ‘Cancel’ button is shown while uninstalling, though it is insensitive because uninstallation operations can’t be safely cancelled
• The ‘Open’ button is styled as suggested-action as otherwise no button has that style and it looks odd
• Similarly for the ‘Reboot’ button, I kept suggested-action styling on it in the end, as otherwise no buttons are suggested and they blend into the background in the header
• The implementation is not adaptive; that’s left to !904 (merged) (which might need rebasing on this once it lands; this MR is more important as it’s part of the design refresh, and adaptive support is not specifically on the roadmap for 41)

Closes #1351 (closed)