Feature/104 ability to add lecture notes

@kvirani @fourmojocto Tada!

Teachers can see lecture notes written in a gist. 

At the moment you have to use the rails console to add lecture note gists to an activity (or we can use our activities_seed.rb file). Not sure if we want to do this through the UI... LMK

See Trello card for more info:
https://trello.com/c/0JpfV6v0/104-ability-to-add-lecture-notes