Bulk-add labels to selected files

[mzur]

In the volume overview, users could bulk-add image/video labels to images by pressing Shift, similar to selecting thumbnails in Largo. First, a label is selected and attached to the first image/video. Then the user presses Shift and selects the "last" image/video. All images/videos between the two get the label as well. This needs a new API endpoint to accept a bulk-update, as an HTTP request for each image will be too much.