integrated labels management for recordings

[kdarras]

This is an alternative to managing labels in the classical way (#234 ), which is now obsolete.

Labels are "just" labels, so it might suffice to display them only in collection views, in a GitHub-style manner, and give power over them to the users.

Correspondingly:

• recordings should have no default label (remove all "not analysed" labels). Thus, there should be no distinction between public and user-generated labels (field can be removed)
• it should be possible to assign multiple labels to recordings
• project managers should see all labels of their managed project, along with the user who assigned them, users would see only their labels
• in each collection view except the timeline view, next to the text search box, a label search box should be added, with the possibility of filtering the collection for multiple labels (combined with OR operand), and an "edit" entry at the bottom to add new labels. After filtering, all the summary statistics of the collection should be correspondingly filtered, and the leaflet map too, to be consistent, if possible.
• when present, labels should be displayed below or next to each recording, and a small "+" should allow users to assign labels to recordings from the collection view rapidly (instead of opening each single recording for assigning the label)

Later, it should be possible to assign labels automatically, like in Github workflows (#246 )