Add API endpoint with teacher list

[pniedzwiedzinski]

/api/teacher

Add a view that will return a list of substitute teachers for the current day.

Useful links

• https://docs.djangoproject.com/en/3.0/ref/models/querysets/#only
• https://stackoverflow.com/questions/4668619/how-do-i-filter-query-objects-by-date-range-in-django
• https://stackoverflow.com/questions/3852104/select-distinct-individual-columns-in-django