Add support for IDN

[tuxmaster5000]

Gitea 1.13.4 will not show email address with an idn domain part correct.
For example the IDN föö.de is shown as the ACE one xn--f-1gaa.de

[noerw]

for reference: https://en.wikipedia.org/wiki/International_email

did you check if the email is stored correctly in DB?

[tuxmaster5000]

Yes the mail is stored as the ace one in the data base. So I think it is stored correct, but shown wrong.
But when I try to add an email in the IDN coding via the ui, then it will fails with the error: invalid email address.

[zeripath]

IDN is a nightmare to do safely.

Any PR attempting to add this will need serious review.