All icons in own repo and included as a submodule (#11548)

* add icons submodule

* replace path

* change submodule url

* fix JSONTest

* fix integration tests

* update icon submodules

* fix preg_match

* fix remaining system tests

* better match for non dist icon files

* fix .gitmodules

* update icon submodule

* Fixed custom dimension test

* Do not completely fail when GD not enabled

* Fix http test + refactor checklist

* Refactored the code into a method

* Changed icon path refs https://github.com/piwik/piwik/pull/11383

* System tests

* Update valid UI tests

* Fix integration test