Add join table between Courses and Users

[hjarrell]

Add a many-to-many relationship between the Users table and the Courses table. Here are some guides that could help:

• https://hexdocs.pm/ecto/Ecto.Schema.html#many_to_many/3
• https://medium.com/coletiv-stories/ecto-elixir-many-to-many-relationships-66403933f8c1
• https://elixirschool.com/en/lessons/ecto/associations/

[hjarrell]

Also add an integer column in the relation for Role.