Rebasing should have a confirmation

[garrett]

Rebasing doesn't have a confirmation.

It just rebases (without proper progress, and is uncancelable) and then instantly reboots when done.

• There should be a confirmation, especially as a reboot is done after rebasing.
• It should have progress. (Or at least a spinner.)
• Ideally it would be able to be cancelled. (I can hit control-c in the middle of a rebase on the command line, so I think it can be cancelled? But it might not be able to be cancelled via API.)

What information can we show in the confirmation modal? Download size? Definitely something about rebooting. Probably what version which will be pulled? And perhaps the date? (Although the version is usually the date, basically.)