Replace ActionHeader minicons with Font icons

Summary:
Ref T5885. See D10276.

Currently, ActionHeaders can only have minicons, and we don't use them anywhere and they probably don't make much sense in the product anymore.

Instead, allow them to have font icons. Remove minicons, which have no callsites and probably won't in the future.

Test Plan:
{F190925}

  - Grepped for `minicons`.
  - Grepped for `setHeaderIcon()`.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5885

Differential Revision: https://secure.phabricator.com/D10277