add redcarpet and markdown method

This will allow us to write review notes using markdown
and then display that when the student sees their feedback.
change Grade belongs_to submission to review

This was left over from the old schema. Grades had
a review_id so Review has_many grades worked, but
grades didn't have access to the review they belonged
to.
add hidden field for requirement id when submitting a review

Without this, grades didn't know what requirement they belonged
to. Now they do.
edit markdown helper method to use custom MarkdownRenderer class

Before I just copied what Michael had done in the textbook app.
Unfortunately that didn't properly use the custom class he had
created to make sure links rendered with redcarpet opened in an
new tab. This fixes that here.
add lastest_review association to submission

I wanted to make sure that when user sees feedback for an assessment,
it is most recent feedback.
update assessment#show to display submission feedback for user

After their submission has been reviewed, a user can visit the
assessment page and see the feedback from the review. They can
still resubmit and see the requirements.