Add sidebar to Horizons site

By March 6th. 

This side bar should allow the user to quickly access sections of the website. The present figma does not contain it, but there is a version of it in the 2020 version seen [here].(https://www.figma.com/file/69oNDDXtOVLagUVcwBOJiV/Event-Site?node-id=219%3A2)