PHP Implementation of OOjs UI

The PHP implementation will include only a subset of elements, widgets
and layouts, based on what makes sense for static content. Future work
will be done to support progressive enhancement on the client for
certain widgets, such as turning a TextInputWidget into an
MwTitleInputWidget.

Working so far:
* ButtonElement
* FlaggedElement
* IconElement
* IndicatorElement
* LabelElement
* ButtonWidget

Change-Id: Ifcc9b859c76e9c3f1d6ca2012a21d4198caa8240