Issue 12 - Handle deleted translations

[djbusstop]

Resolves #12

This PR:

• Adds deleted_at to Translation object
• deleted_at is set on Translations when they are removed from projects, identified in sync_project
  • Creates new method mark_deleted_translations which has test
• Any translation that has deleted_at set will be excluded from the generated locale files output
  • This is tested
• filter_latest_translations() is modified to only return the non-deleted Translations by default
• Changes admin interface:
  • Link on project rows, navigates to all translations for that project.
  • Shows if a translation has been deleted on a translation row.
  • Readonly fields for created and deleted time for translation
• /translations/ endpoint only returns translations that have not been deleted
  • Therefore, editors can only edit translations that haven't been deleted