Implement HSTS preload

This allows browsers to query https directly instead of the initial http redirect

https://hstspreload.org/#opt-in says it should be explicitly opt in

dashboard/src/views/app.html

@@ -1282,6 +1282,15 @@
                 <textarea ng-model="security.csp" placeholder="default-src 'self'; frame-ancestors 'none';" class="form-control text-monospace" rows="2"></textarea>
               </div>
 
+              <div class="form-group">
+                <div class="checkbox">
+                  <label>
+                    <input type="checkbox" ng-model="security.hstsPreload">{{ 'app.security.hstsPreload' | tr }}</input>
+                    <sup><a ng-href="https://docs.cloudron.io/apps/#hsts-preload" class="help" target="_blank"><i class="fa fa-question-circle"></i></a></sup>
+                  </label>
+                </div>
+              </div>
+
               <input class="ng-hide" type="submit" ng-disabled="securityForm.$invalid || security.busy"/>
             </form>
           </div>