Restore display of abstract length requirements

Accidentally lost during 81853d1ef9

Resolves:

  1) Event as a participant not signed_in user submits proposal
     Failure/Error: expect(page).to have_text 'Abstracts must be between 0 and 500 words.'
       expected to find text "Abstracts must be between 0 and 500 words." in "…Abstracts must be between 0 and 250 words.…"
     # ./spec/features/proposals_spec.rb:94

250 is the initial value hard-coded in app/views/proposals/_form.html.haml

app/helpers/event_types_helper.rb

@@ -8,6 +8,13 @@ module EventTypesHelper
   # ====Returns
   # * +String+ -> number of registrations / max allowed registrations
   def event_type_select_options(event_types = {})
-    event_types.map { |type| ["#{type.title} - #{show_time(type.length)}", type.id] }
+    event_types.map do |type|
+      content = "#{type.title} - #{show_time(type.length)}"
+      value = type.id
+      attributes = { data: { min_words: type.minimum_abstract_length,
+                             max_words: type.maximum_abstract_length } }
+
+      [content, value, attributes]
+    end
   end
 end