R1:98884f758a29

Fix call to non-existing newIcons() in PhabricatorIconSet

Summary:
`PhabricatorIconSet::getIcons()` calls `$this->newIcons()` which is undefined.

As all six child classes which extend `PhabricatorIconSet` implement `newIcons()`, declare an abstract function in their parent class.

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision…