Purpose of colorful stripes in session cells is unclear

Colorful stripes in lists show which track a session belongs to.

## Issue
When there are many tracks, it is unclear which color is associated with which track.

## Solution proposal
### Tracks page
Instead of being part of the session cells, a colorful element can be part of the track header (1) to make it clear that a color represents a specific track.

### Schedule page
"Browse session" lists are currently sorted alphabetically. Sort them first by track, then alphabetically, remove the colorful stripe from the session cells and include headers (like 1) with the track name and color instead.

### Session page
The name of the track should be showed along with the track color in a style similar to the header used on the tracks and schedule pages.