Add RSS feed

[ricco386]

Use Django syndication feed framework to build RSS feed.

For this new classes create feed.py file. We can have multiple feeds:

• Feed for all our courses -> this will notify subscribed users if new course is added
• For each course run which will have all the weeks (include meetings) -> this will be used for iCal generation (in next step)
• For each course run just the meetings -> this will be used for iCal generation (in next step)
• For each course run which will have just open lectures -> this will notify subscribed users if new lecture is open

Please add a tests for the views that dispaly data as they should in the RSS.

Second step would be build iCal support, so people can add the course run to their calendars. For that use: https://django-ical.readthedocs.io/en/latest/

Feel free to add opinion for another usefull RSS or iCal...