Automatically focus first autocomplete result

In order to do this space-to-complete needs to be removed too since it
doesn't work together with autofocus. I think this is a good thing
anyway as it can cause accidental selections when typing. And we already
have two other shortcuts for this too (enter and tab).

app/assets/javascripts/autocomplete.js

@@ -46,6 +46,7 @@
 
     $fields_multiple.autocomplete({
       delay: 100,
+      autoFocus: true,
       focus: function() {
         return false;
       },
@@ -148,6 +149,7 @@
 
     $fields_single.autocomplete({
       minLength: 1,
+      autoFocus: true,
       source: function(req, resp) {
         Danbooru.Autocomplete.normal_source(req.term, resp);
       }

vendor/assets/javascripts/jquery-ui-autocomplete-custom.js

@@ -116,7 +116,6 @@ $.widget( "ui.autocomplete", {
           suppressKeyPress = true;
           this._keyEvent( "next", event );
           break;
-        case keyCode.SPACE:
         case keyCode.ENTER:
         case keyCode.NUMPAD_ENTER:
           // when menu is open and has focus