Misc fixes instructor & student list query and quiz link

[brianhogg]

Description

Removes the "quiz" link from lessons since the quiz resource doesn't exist.

Sets the default listing of the students and instructors endpoints to be role "student" and "instructor" by default. It can still be changed using the roles query param.

Refs #48 Fixes #330

How has this been tested?

Manually

Screenshots

Checklist:

• [ ] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.