Improve email confirmation flow after registration

Summary

According to some analysis done by @tszuromi, some users are getting stuck in the confirmation step when email registration is required. This is seen by a loop that looks like:

• /users/confirmation/new -> /users/almost_there -> /users/confirmation/new -> /users/almost_there -> /users/confirmation/new -> /users/almost_there -> /users/confirmation/new -> /users/almost_there -> /users/confirmation/new -> /users/almost_there

We should diagnose this problem and fix it.

Steps to reproduce

Currently if you don't get a confirmation email you can try to send it multiple times.

Proposal

TBD