Display track description on proposal form

Tracks are otherwise undocumented for CfP respondents.

app/views/proposals/_form.html.haml

@@ -19,7 +19,9 @@
   - if @program.tracks.confirmed.cfp_active.any?
     .form-group
       = f.label :track_id, 'Track'
-      = f.select :track_id, @program.tracks.confirmed.cfp_active.pluck(:name, :id), { include_blank: '(Please select)' }, { class: 'form-control' }
+      = f.select :track_id, @program.tracks.confirmed.cfp_active.pluck(:name, :id), { include_blank: '(Please select)' }, { class: 'form-control select-help-toggle' }
+      = render 'shared/select_help_text', f: f, for: :track_id, include_blank: true, options: @program.tracks.confirmed.cfp_active do |track|
+        = markdown(track.description)
 .form-group
   = f.label :event_type_id, 'Type'
   = f.select :event_type_id, event_type_select_options(@conference.program.event_types), { include_blank: false }, { class: 'select-help-toggle form-control' }

spec/features/proposals_spec.rb

@@ -106,6 +106,9 @@ feature Event do
 
     scenario 'update a proposal', js: true do
       conference = create(:conference)
+      create :track, program:     conference.program,
+                     name:        'Example Track',
+                     description: 'This track is an *example*.'
       create(:cfp, program: conference.program)
       proposal = create(:event, program: conference.program)
 
@@ -114,6 +117,8 @@ feature Event do
       visit edit_conference_program_proposal_path(proposal.program.conference.short_title, proposal)
 
       fill_in 'event_subtitle', with: 'My event subtitle'
+      select 'Example Track', from: 'Track'
+      expect(page).to have_selector '.in', text: 'This track is an example.'
       select('Easy', from: 'event[difficulty_level_id]')
       expect(page).to have_selector '.in', text: 'Events are understandable for everyone without knowledge of the topic.'