Fix group aliases causing private messages to not be created

Bug:
 - Open the private message in discourse
 - in the user selector, select a user group you are part of
 - send the message

What should happen:
 - the private message should be created

What happens:
 - the user is thrown a weird "Archetype" error, stating the user isn't able to send to this user:

![screen shot 2014-02-21 at 11 11 05](https://f.cloud.github.com/assets/40496/2228909/a3d642e4-9ae3-11e3-8a4d-341d82cd08c2.png)
(The writing user here is "eviltrout").

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This Pull-Request fixes that issue by applying the exclusion of the own username as well as already selected usernames to group aliases, too. Therefore preventing to accidentally selecting yourself by choosing a group you are part of and - as a side effect - prevent double selecting of users.