Improve performance of Move Topic

As you can see from here:

        Branch targetBranch = branchDao.get(branchId);
        targetBranch.addTopic(topic);
        branchDao.saveOrUpdate(targetBranch);

We add a topic to the branch which shouldn't happen. The mapping should be inverse=true so that topic saves a link to the branch. Right now the saving of the branch may take awhile because it's probably fetching all the topics from DB.

If you can find other improvements of Move Topic, feel free to fix them as well.