MWLinkTargetInputWidget: Add styles for invalid link titles

This color is chosen because it is already present as a warning color in
other mediawiki elements like remove button in Link Inspector. A font
weight of bold is also given to make it more prominent.

Things done for this commit:
* A new CSS file 've.ui.MWLinkTargetInputWidget.css' is added.
* Warning text style is added to the class
  '.ve-ui-mwLinkTargetInputWidget-warning' in this file.
* The CSS file is included in the style resources in the
  file 'VisualEditor.php'.

Tested in latest stable firefox, chrome in Ubuntu 14.04.

Bug: 62761
Change-Id: I89e96e0444f8505123f3c653cf2dd6b74fc83623