Merge doc and change searches.

This CL puts a dropdown in the SearchPanel, to choose from changes
search (default) or documentation search.

I mostly copied the css for the search button to the dropdown, with
some minor changes.

If docs are not available (e.g. built without docs), the whole dropdown
will be gone and it should be the same as before.

Screenshot: https://i.imgur.com/Qyg2VZg.png

Change-Id: I04e15327f43fc9c8002cba364b44d77f52075234

gerrit-server/src/main/java/com/google/gerrit/server/documentation/QueryDocumentationExecutor.java

@@ -74,7 +74,7 @@ public class QueryDocumentationExecutor {
   }
 
   public List<DocResult> doQuery(String q) throws DocQueryException {
-    if (parser == null || searcher == null) {
+    if (!isAvailable()) {
       throw new DocQueryException("Documentation search not available");
     }
     try {
@@ -123,6 +123,10 @@ public class QueryDocumentationExecutor {
     return dir;
   }
 
+  public boolean isAvailable() {
+    return parser != null && searcher != null;
+  }
+
   @SuppressWarnings("serial")
   public static class DocQueryException extends Exception {
     DocQueryException() {