Capping sortation criteria to 3.
Merge pull request #1469 from opf/feature/max-sort-criteria-9834

Capping sortation criteria to 3.
Restrict selectable sorting options to those not selected yet


TODO: Add controller test
Add unit test for sorting modal
Add unit test for selectable sort option filtering
Merge branch 'feature/round-off-sorting-dialog-8280' into feature/column-sortation-tightening-up-9836
Add unit test for sort option filtering in sorting modal
Merge branch 'feature/column-sortation-tightening-up-9836' of github.com:opf/openproject into feature/column-sortation-tightening-up-9836

Conflicts:
	app/assets/javascripts/angular/controllers/dialogs/sorting.js