Give the search page a little UI love

Uses mediawiki.ui to make the input field more legible and modern,
as well as giving the search button a lovely shade of blue.

Screenshot: https://www.mediawiki.org/wiki/File:Search-UI-minor-tweak.png
Change-Id: I7fdd9970ed5807df94c04bec1681f33523cad173
Merge "Give the search page a little UI love"