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
Also, in events#show and proposals#index
It is more relevant to check if the speakers have registered to the
conference and filled their biographies than the submitter, because,
there can be multiple speakers and the submitter isn't necessarily one
of them
And, remove admin/events/reports.html.haml, since, it isn't used anymore
Fix#1477, fix#1479
