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: https://we.phorge.it/D25647