Matching pages from suggestions in link dialog now actually work as links

* A subclass of MenuItemWidget from OOUI, MWLinkMenuItemWidget, is introduced that, when used in MWLinkTargetInputWidget for both
external and internal links, returns actual page links.

* MWInternalLinkMenuItemWidget is extended from MWLinkMenuItemWidget to support the internal links.

* LinkCache is being used in MWInternalLinkMenuItemWidet to stylize the links. Thus, getting rid of some old css classes.

Bug: 51205
Change-Id: I3ac18dabd514ca539fff1db3c67ae97931c3d4fc
Merge "Matching pages from suggestions in link dialog now actually work as links"