feature/home-screen

This PR implements the home screen functionality, which displays all the courses and decks. 

Notes:
- Reorganized the templates to follow the convention that extendable templates are prefixed by two underscores, includable templates are prefixed by one underscore.
- Added CSS and Javascript blocks to the base template.
- Added a left sidebar layout template that is extendable.
- Added a collection nav includable template.
- Implement the collection/deck display.