Improve debounce functionality of REST requests

Follow up on https://github.com/syncthing/syncthing/pull/711
I found a way to make it cleaner and added a 'force' flag. I'm unsure about the correct timing to force the request, is this:
```
 303             $scope.repos[repo].Nodes.forEach(function (nodeCfg) {
 304                 refreshCompletion(nodeCfg.NodeID, repo);
 305             });
```
and change it to something like this:
```
..., repo, _total == 100)
```
?