Convert workboard column options into a dropdown menu

Summary:
Ref T5024, T4427, T5474, T5523. Instead of separate icons in the column header for "Create Task" and "Edit Column Settings", use a dropdown menu.

  - T5024 will likely add a "View Standalone" option.
  - T4427 needs header space to show a count.
  - T5474 likely needs "Edit Triggers..." (this seems reasonable to separate from editing the name, etc.)
  - T5523 likely adds "Move all tasks..." eventually.

Test Plan: {F187414}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5523, T5474, T5024, T4427

Differential Revision: https://secure.phabricator.com/D10190