Added method removeAllowedUser to topic_details.js

perform a request to /t/:id/remove-allowed-user,
then delete the user from the local list
Improved private message user removal, locale edit

replaced array iteration with findProperty (will be renamed and aliased to findBy with Ember 1.1)
added modal dialog to verify
catched nil-error on topic.rb
Merge pull request #1488 from cfstras/removeAllowedUser

Allow removing users from private conversations