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/factories/users.rb

@@ -29,6 +29,7 @@ FactoryGirl.define do
       Quisque cursus facilisis consequat. Etiam volutpat ligula turpis, at
       gravida.
     EOS
+    is_disabled false
 
     after(:create) do |user|
       user.is_admin = false
@@ -44,6 +45,10 @@ FactoryGirl.define do
         user.save!
       end
     end
+
+    trait :disabled do
+      is_disabled true
+    end
   end
 
   factory :user_xss, parent: :user do