Make determinant equation easier to read

Summary:
Allow matrix input to include solution boxes outside of the matrix input

New exercise type where one of two matrices is non-invertible

Test Plan: Make sure all the exercise types works. Also try some of the other matrix exercise to make sure I've not broken them.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D11448