web: Show a 'Clone now' button in repos list if not cloned yet

Created by: mrnugget

This fixes #4232 (closed) by making it clearer to the user that they can speed up the cloning process by clicking on the "Clone now" button.

We (@keegancsmith and I) discussed other solutions (showing the position in the clone schedule, rewording "Not yet cloned" label, etc.) but ultimately came to the conclusion that "Not yet cloned" is correct and that showing any position in a cloning-schedule would be unreliable.

"Clone now" on the other hand makes the tooltip text of the "Not yet cloned" label actionable as a button and gives the user something to do.

This is what it looks like now: