Fold task-relationship actions into an accordion dropdown

Summary:
Ref T11179. Alternative to D16152. I think this turned out a bit better than the other one did.

Currently, we render two copies of the menu (one for mobile, one for desktop). A big chunk of this is sharing the nodes instead: when you open the mobile dropdown menu, it steals the nodes from the document. When you close it, it puts them back. Magic! Sneaky!

Test Plan:
{F1695499}

{F1695500}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11179

Differential Revision: https://secure.phabricator.com/D16157