WET theme: Switched site search to using Google Web search

@nschonni This change would allow the v3.1 site search to work with Google Web search (if we go that way for search and if the action is changed to https://google.ca/search). Should this change be propagated to the other themes? It is a markup change but not a mandatory one (besides people are expected to configure the form method and field name anyway so it effectively would be non-breaking).