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

spec/models/user_spec.rb

@@ -10,6 +10,7 @@ describe User do
   let(:volunteers_coordinator_role) { Role.find_by(name: 'volunteers_coordinator', resource: conference) }
   let(:organizer) { create(:user, role_ids: [organizer_role.id]) }
   let(:user) { create(:user) }
+  let(:user_disabled) { create(:user, :disabled) }
 
   let(:event1) { create(:event, program: conference.program) }
   let(:another_conference) { create(:conference) }
@@ -75,6 +76,16 @@ describe User do
       end
     end
 
+    describe '.active' do
+      it 'includes users without is_disabled flag' do
+        expect(User.active).to include(user)
+      end
+
+      it 'excludes users with is_disabled flag' do
+        expect(User.active).not_to include(user_disabled)
+      end
+    end
+
     describe '.comment_notifiable' do
       let(:cfp_user) { create(:user, role_ids: [cfp_role.id]) }