When the "assign" button is clicked on the "assign labels" modal
- the form should submit
- on submission the selected labels should be assigned to the selected watchlist items
- then reload Special:EditWatchlist in the same place in the pagination
- show a message saying something like "X labels assigned to Y pages" (but check with design/product for exact wording)
The new form submission parameters are watchlistlabels[] (an array of IDs) and watchlistlabels=assign|unassign (the action to take), as well as the existing wpTitles[] array. For example:
{ "wpEditToken": "…", "limit": "50", "watchlistlabels[]": ["21","33"], "watchlistlabels-action": "assign", "wpTitles[]": ["Lorem", "Ipsum"] }