Add helper function for speaker selection

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

app/models/user.rb

@@ -53,6 +53,7 @@ class User < ActiveRecord::Base
   accepts_nested_attributes_for :roles
 
   scope :admin, -> { where(is_admin: true) }
+  scope :active, -> { where(is_disabled: false) }
 
   validates :email, presence: true