Token deletion with simple undo

Following the lines of
- http://alistapart.com/article/neveruseawarning
- https://github.com/owncloud/core/issues/9268
- https://github.com/owncloud/contacts/issues/107

I've added simple undo functionality to the "delete token" action.
The user has 5 seconds to click on the "restore" button to undo the
token deletion.