Add support for retrieving config for outbound messages from RapidSMS.
Remove old TODOs.
Construct vumi_username correctly in RapidSMS vumi_app.
Update comment explaining vumi_username value.
Add test for vumi_username_for_conversation.
Merge branch 'feature/issue-835-fix-rapidsms-get-config' into develop
Merge branch 'develop' of github.com:praekelt/vumi-go into develop