Improve guardian flow by adding "done" and/or "find class" button after student registration

[rgroves]

For a guardian, after successfully registering a student, the flow stays on the register student form. It would be a nice UX addition to have one, or both, of the following options on that same page: a "Done" option that takes you back to the account screen or a "Find Classes" option that takes you to the class list to register.

Originally posted by @rgroves in https://github.com/WeAllCode/website/issues/820#issuecomment-763376028