Fix: register when logged in

[parisiam]

Fixes #294

• Prevents a logged in user to register
• A corresponding unit test was added

[MGatner]

Can we get the same on registerAction()?

[parisiam]

Yes, sure, happy to help ;)
I've submitted the changes for registerAction() and for the tests.
Please check the tests (I may have be redundant or incomplete).

[kenjis]

What if a user is pending login state (in the process of 2FA)?
See #132

[MGatner]

I think that's fine. They may have mistyped their email address, or want multiple accounts. In my opinion this is mostly about preventing confusion, rather than a security or behavior issue.

[kenjis]

Okay, thanks!