UX: Better placement for bulk select actions button (PR #14419)

The bulk select action button is placed awkwardly on the right of the screen. This PR makes the table header for these lists sticky, and places that button there alongside the other buttons (toggling bulk select, select all, select none).

Note that some plugins and themes use the bulk-select-button component, PRs incoming.




(This could be canDoBulkActions: and("currentUser.staff", "selected"), but I leave it up to you :wink:)

That is much better, thank you. (In fact, this wasn’t working correctly in qunit, and it took me a very long time to figure out I needed to check for selected.length. It was fine under Ember CLI.)