refacto + distinct in content migration

...rollers/admin/communication/blocks/headings_controller.rb

app/controllers/admin/communication/blocks_controller.rb

app/models/communication/block.rb

app/models/communication/block/heading.rb

app/models/concerns/with_blocks.rb

app/services/content_migration.rb

...views/admin/communication/blocks/content/_editor.html.erb

app/views/admin/communication/blocks/content/_show.html.erb

...views/admin/communication/blocks/content/_static.html.erb

...ews/admin/communication/blocks/headings/_heading.html.erb

app/views/admin/communication/blocks/headings/_show.html.erb