waterfall config fine-adjustments

* hide the waterfall colors input when pre-defined color scheme is
  selected
* skip unparseable lines on custom color input
* fallback to black and white if custom color config is unusable
* always use the waterfall classes when sending changes to the client

htdocs/lib/settings/WaterfallDropdown.js

@@ -0,0 +1,11 @@
+$.fn.waterfallDropdown = function(){
+    this.each(function(){
+        var $select = $(this);
+        var setVisibility = function() {
+            var show = $select.val() === 'CUSTOM';
+            $('#waterfall_colors').parents('.form-group')[show ? 'show' : 'hide']();
+        }
+        $select.on('change', setVisibility);
+        setVisibility();
+    })
+}

htdocs/settings.js

@@ -4,4 +4,5 @@ $(function(){
     $('.imageupload').imageUpload();
     $('.bookmarks').bookmarktable();
     $('.wsjt-decoding-depths').wsjtDecodingDepthsInput();
+    $('#waterfall_scheme').waterfallDropdown();
 });