Fix template segment counts

[cavis]

The only place audio_version_templates.segment_count was being set was the feed importer. So we weren't running seg-count validations.

• [x] Reset segment_count every time a child AudioFileTemplate changes
  • We were already touching the timestamp field, so this change keeps doing that but also checks the AudioVersionTemplate.audio_file_templates.count
  • You can still manually use/set the segment_count field, IFF you have no child file_templates
• [ ] Add the test_after_commit gem so these after_commit hooks actually run in the tests. (database_cleaner usually prevents them).
• [x] Add the m gem because it's convenient
• [x] Send the story released_at date in the initial feeder POST episode
• [x] Add segment_count to audio_version and audio_version_template representers