Batch multi-selection actions into single requests

[frankrousseau]

Problem

• Many multi-selection actions fired one HTTP request per selected item (or per production), N requests where a single one suffices: clearing a person's assignation on a selection, the schedule auto-distribution, creating an entity's default tasks, adding selections/episodes/movies to a playlist, deleting a selection of entities, saving casting on a selection, and changing task priority on a selection.
• runPromiseAsSeries received already-created promises, so its call sites fired in parallel despite reading as serialized; the "add movie" playlist path also rewrote the whole playlist once per shot, racing against concurrent edits.

Solution

• Clear a person's assignation on a selection, change priority on a selection, and create an entity's default tasks now each use a single request (via the existing clear-assignation route, the new set-priority route, and the entity tasks route respectively).
• Batch the schedule auto-distribution's assign/unassign into one request per assignee instead of one per task.
• Add a selection, an episode or a whole movie to a playlist, delete a selection of assets/shots/edits/concepts, and save casting on a selection now each use one request via the new zou batch routes.
• Replace runPromiseAsSeries with real serialization (runPromiseMapAsSeries), which creates each promise only after the previous resolves, and drop the misleading helper.
• The playlist, delete-entities, casting and set-priority parts depend on the new routes in Add batch action routes for multi-selection Kitsu operations zou#1145.