Add generic support for search box in page header

This adds the ability to show a search box in any pageheader with correct href and
hidden form data, but does not enable the box on any pages.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.css

@@ -61,6 +61,11 @@ div#header {
 div#header img#logo {
 	float: right;
 }
+
+div#header input {
+	float: right;
+	margin: 0.25em 1em;
+}
 div#header a {
 	color: black;
 }