Add mw-ui-input to mediawiki ui

Upstream component from Flow
Adds mw-ui-input component
Update existing (and undocumented) usage in core of mw-ui-input to
use Flow focus style

Updated usage of mw-ui-input in Special:Search and Special:Userlogin
Packaged in mediawiki.ui.inputs to allow developers to use inputs
separately to other mediawiki ui components.

Change-Id: Ida765b97e6574bdc8fbba88a08ec98ec12f3dc95
Merge "Add mw-ui-input to mediawiki ui"