Don't automatically select difficulty level

Resolves accidental selection of first difficulty level when intending
to edit only other fields.

Author: AndrewKvalheim

app/views/proposals/_form.html.haml

@@ -38,7 +38,7 @@
 - if display_details
   - if @conference.program.difficulty_levels.any?
     = f.label :difficulty_level
-    = f.select :difficulty_level_id, @conference.program.difficulty_levels.map{|level| [level.title, level.id ] }, {include_blank: false}, { class: 'select-help-toggle form-control' }
+    = f.select :difficulty_level_id, @conference.program.difficulty_levels.map{|level| [level.title, level.id ] }, {include_blank: '(Please select)'}, { class: 'select-help-toggle form-control' }
     = render 'shared/select_help_text', f: f, for: :difficulty_level_id, options: @conference.program.difficulty_levels do |difficulty_level|
       = difficulty_level.description
 .form-group