cmd/syncthing: Refactor out staticsServer (prev. embeddedStatic) a bit

The purpose of this operation is to separate the serving of GUI assets a
bit from the serving of the REST API. It's by no means complete. The end
goal is something like a combined server type that embeds a statics
server and an API server and wraps it in authentication and HTTPS and
stuff, plus possibly a named pipe server that only provides the API and
does not wrap in the same authentication etc.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3273

gui/default/syncthing/settings/settingsModalView.html

@@ -141,10 +141,10 @@
               </button>
             </div>
 
-            <div class="form-group" ng-if="system.themes.length > 1">
+            <div class="form-group" ng-if="themes.length > 1">
               <label>GUI Theme</label>
               <select class="form-control" ng-model="tmpGUI.theme">
-                <option ng-repeat="theme in system.themes.sort()" value="{{ theme }}">
+                <option ng-repeat="theme in themes.sort()" value="{{ theme }}">
                   {{ themeName(theme) }}
                 </option>
               </select>