Add info box for email confirmation and sign-ups disabled.

[MaxBroome]

Improved the sign-up page with a couple of little tweaks 😉

1. If sign-ups were disabled in Supabase I added a specific alert box for that, rather than the form throwing a generic error to the user. That ways an admin can disable sign-ups directly in the DB if they have too much demand; and users know to come back later.
2. When signing up, after a user enters their desired email and password, the page automatically tried to forward them to the dashboard, however most Supabase instances have email verification first before a user is allowed to sign-in. So this would fail and just redirect them to the login page with no messages (a very similar looking page, and some users might even think that they are trying to sign-up again as it failed.).

However even though I didn't implement it, if some people want to disable email verification then this would be confusing. I just realized this after I submitted this PR. Maybe an option to say if they have verification enabled or disabled could be in order; but that's a later task.

3. Small fix but 'Email' was misspelled as 'Emial' on both forms. 😄

[kizivat]

We can check if session returned from signUp is non-null. If so, we can still redirect.

[kizivat]

Thank you @MaxBroome, that is a very good point!

I fixed the issue with disabled email verification as well (the signUp() method returns non-null session right away then).

I wanted to add it to this PR as I see you've allowed edits from maintainers, but somehow my push to your fork still fails, so I'll add it afterwards. Feel free to check it out then. :)