Create new Django app kolibri_public

[bjester]

Parent: #3808

Desired behavior

• A new Django app called kolibri_public should be added to Studio's codebase for housing Kolibri-specific public APIs
• The new app's API URLs should be organized under the path /kolibri/api/public
• The app should route database queries to Studio's postgres database like contentcuration
• The app should add the header Cache-control: public to all API responses
  • Suggestion: create a base viewset class/mixin which uses finalize_response to add the header and defines a method that can be overridden to return a 'last modified' date

Current behavior

We have an existing kolibri_content app that is used for generating channel data during publishing

Value add

This Django app will house public Kolibri APIs that will support remote content browsing on Studio

[rtibbles]

This was completed in #3941