The line '@users = User.all.order(:name)' is replicated a lot of times
in EventsController and ProposalsController.
So, this commit removes it and adds a helper function
'speaker_selector_input' that generates the field where the @users
variable was used.
Also, it makes the query more specific.
Fix#1455
Other changes:
* Include the username in the drop down menu
* Add .active scope to User and corresponding tests
* Add :disabled trait to User factory
