Translate transit_indicators choices classes in django

These translations are done in Django for the following reasons:
  - Reduce coupling between the angular/django apps
  - Reduce amount of copy/pasting and repeated string definitions
    in the angular app.

Other options explored for translating these choices strings included:
  - Sending an angular-translate key as the django human-readable text
    and translating client side
  - Wrapping django/angular translations in a management/grunt command
    that would combine to a single po file to be sent to translators
Merge pull request #126 from WorldBank-Transport/feature/i18n-django-choices

Translate transit_indicators choices classes in django