Split join page for personal and group accounts

[ja5087]

We have a minority of cases where users try to sign up for group accounts using their personal CalNet ID association and vice versa. Splitting up this page would reduce a lot of confusion. My suggestion would be to create a page that splits up like Create Account: [User] | [Group] explaining the differences. Also, we could handle other account creation errors here.

Context here: https://irclogs.ocf.berkeley.edu/rebuild/2020-07-28#856949

Register page code: https://github.com/ocf/ocfweb/blob/master/ocfweb/account/register.py