Repo label query

Implement the selection of repositories by label. This also required a complete overhaul of how repo selection is done in the frontend: Instead of providing a simple drop-down list which selects+closes on click, we now have a separate internal window which opens and provides two selection fields: one for selection by name/ID and for selection by possible labels. They are mutually exclusive to be in line with the backend, though we might also add selection of multiple mirrors, which is not yet done. In addition we have some nice restyling to make the lists look good.