Simplify broadcast message JS

Also resets the default example message when the user input is blank.

app/assets/javascripts/admin.js.coffee

@@ -13,15 +13,16 @@ class @Admin
       $('.js-toggle-colors-container').toggle()
 
     $('input#broadcast_message_color').on 'input', ->
-      previewColor = $('input#broadcast_message_color').val()
+      previewColor = $(@).val()
       $('div.broadcast-message-preview').css('background-color', previewColor)
 
     $('input#broadcast_message_font').on 'input', ->
-      previewColor = $('input#broadcast_message_font').val()
+      previewColor = $(@).val()
       $('div.broadcast-message-preview').css('color', previewColor)
 
     $('textarea#broadcast_message_message').on 'input', ->
-      previewMessage = $('textarea#broadcast_message_message').val()
+      previewMessage = $(@).val()
+      previewMessage = "Your message here" if previewMessage.trim() == ''
       $('div.broadcast-message-preview span').text(previewMessage)
 
     $('.log-tabs a').click (e) ->