Limit signups from @cps.edu emails

[karbassi]

Is your feature request related to a problem? Please describe.
CPS (Chicago Public Schools) gives all students a @cps.edu email address. Rightly so, it does not allow external emails to students' inbox. Because of the mailbox block our emails about the class does not go through.

Describe the solution you'd like
When someone tries to sign up via a @cps.edu email address, give them an error and suggest they use their personal or guardian's email address.