gui: Disable rescan button while scanning (fixes #4977) (#4979)

gui/default/index.html

@@ -461,7 +461,7 @@
                   <button type="button" class="btn btn-default btn-sm" ng-click="restoreVersions.show(folder.id)" ng-if="folder.versioning.type">
                     <span class="fas fa-undo"></span>&nbsp;<span translate>Versions</span>
                   </button>
-                  <button type="button" class="btn btn-sm btn-default" ng-click="rescanFolder(folder.id)" ng-show="['idle', 'stopped', 'unshared', 'outofsync'].indexOf(folderStatus(folder)) > -1">
+                  <button type="button" class="btn btn-sm btn-default" ng-click="rescanFolder(folder.id)" ng-disabled="['idle', 'stopped', 'unshared', 'outofsync'].indexOf(folderStatus(folder)) < 0">
                     <span class="fas fa-sync"></span>&nbsp;<span translate>Rescan</span>
                   </button>
                   <button type="button" class="btn btn-sm btn-default" ng-click="editFolder(folder)">