Remove "Act As User" local feature, use masqueraded user from LMS

[jonespm]

Thank you for contributing to this project!

Describe your problem or feature you'd like added

We have a feature where you can go into Django Admin and if the user exists act as the user. I think it would make more sense to people if we just used the user that was being masqueraded from Canvas instead. We also wouldn't depend on this page anymore and could probably get closer to not having to store users in the database. (But this is future)

Describe the solution you'd like

Remove the local "Act As User" and user the masqueraded user Id from the Canvas launch.