Ellipsize long labels in Add Action area to avoid text overlap

Summary:
Add CSS to ellipsize long labels and disable text wrapping

Closes T15707

Test Plan:
Set a long custom "action" label in /config/edit/maniphest.points/; go to a Maniphest task; open all items in the "Add Action..." dropdown.

Example config:

{
  "enabled": true,
  "label": "Points",
  "action": "Set Points Yeah Points Points Much Points"
}

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15707

Differential Revision: https://we.phorge.it/D25511