Underline all links

Our links are red, making them difficult for red-green colorblind users to see.
This commit makes all links underlined so that RG-colorblind users can still
easily distinguish them.
Instructors can now be assigned to projects in DB

The DB schema has been modified so that instructors can be assigned to
multiple projects, and projects can be assigned to multiple instructors. There
hasn't been any front-end work to make this possible for the user yet.

I'm going on a limb by making the instructor-project association a
has_many_and_belongs_to_many relationship. The task I'm working on specifically
says, "Assign instructors to projects," suggesting that projects have_an
instructor. However, I can see likely use cases where two or three instructors
take part in the same project, and an instructor decides he or she wants to
help with more than one project. So I believe that the has_many_and_belongs_to_
many association better meets the clients' requirements, but I might be wrong,
and we might need to make changes to this set-up in the future.