Rework how cloning/pulling progress is presented

[krassowski]

Currently the progress messages are just dumped into the cards:

This is bad UX. We should:

• get an actual progress estimate from git (for git clone there are four steps, each of these has 0-100 progress), which can be obtained by using gitpython (see https://stackoverflow.com/a/71285627/6646912) and replacing GitPuller.initialize_repo
• show a subtle progress bar under or over the tile
• maybe put the detailed messages in a notification which would get automatically closed.

[kcpevey]

This sounds like a good approach to me.