Allow track submitter to request specific dates
Redirect to Tracks#edit if a track doesn't have a room or start/end date
before accepting it
Don't allow the submitter or the track organizers to edit the request
after it has been accepted or confirmed
Restrict track selection in proposals and move track selection from
Proposals form to events helper
Mark cfp_active of the tracks table as not null and fill in true if nil
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
