feature request: sync fork button #180

Presently, in order to sync a fork with it's upstream branch on Tangled, you need to run the merge command from the git command line. This is, to be clear, fine for now - but it would be a nice QOL feature to have a "sync fork" button akin to how GitHub has. (related: it'd also be nice to have a quick "submit pr" / "contribute" button for recently pushed to branches on forks, which is right next to the the sync button on GH).