feat(playlists): act on the selected rows from the toolbar, and undo either

While rows are selected the toolbar's second row is the band: the count,
the last target as one click, Move to, Remove, Clear selection. Both write
in one request and undo back to the places; a video the target held before
stays there. Delete takes the selection when there is one. A failure puts
the rows back ticked.

A row's own move goes through the same write, so it undoes too, and the
single-row move route and service function are gone. A playlist row drops
its grip below a 24rem table: the row menu keeps the moves.

frontend/src/i18n/locales/en/playlists.json

@@ -94,7 +94,6 @@
     "sync_all_busy": "Your playlists are already syncing with YouTube — try again when it finishes"
   },
   "moveErrors": {
-    "not_in_playlist": "That video isn't in this playlist any more — it changed somewhere else",
     "same_playlist": "That video is already in this playlist"
   },
   "railMenu": "Playlist actions",
@@ -104,6 +103,24 @@
   "dragHandleOf": "Reorder “{{title}}”",
   "selectRow": "Select “{{title}}”",
   "selectAllShown": "Select all the rows shown",
+  "selectedCount_one": "{{count}} selected",
+  "selectedCount_other": "{{count}} selected",
+  "selectedHidden_one": "{{count}} more hidden by the filter",
+  "selectedHidden_other": "{{count}} more hidden by the filter",
+  "clearSelection": "Clear selection",
+  "moveSelected": "Move to",
+  "removeSelected": "Remove",
+  "movedManyToast_one": "{{count}} video moved to “{{name}}”",
+  "movedManyToast_other": "{{count}} videos moved to “{{name}}”",
+  "removedManyToast_one": "{{count}} video removed from the playlist",
+  "removedManyToast_other": "{{count}} videos removed from the playlist",
+  "moveNoneLeft": "Nothing moved: they aren’t in this playlist any more",
+  "removeNoneLeft": "Nothing removed: they aren’t in this playlist any more",
+  "moveManyFailed": "Couldn't move the videos",
+  "removeManyFailed": "Couldn't remove the videos",
+  "undoMoveFailed": "Couldn't move the video back",
+  "undoMoveManyFailed": "Couldn't move the videos back",
+  "undoRemoveManyFailed": "Couldn't put the videos back",
   "dragBlockedSort": "Reordering by hand is off while a column sort or grouping is applied.",
   "dragBlockedFilter": "Reordering by hand is off while a filter is applied.",
   "clearView": "Reset view",