Redesign of email settings (#2426)

Co-authored-by: Felix Rindt <felix@rindt.me>

src/pretix/control/templates/pretixcontrol/email/email_setup.txt

@@ -0,0 +1,14 @@
+{% load i18n %}{% blocktrans with code=code instance=instance %}Hello,
+
+someone requested to use {{ address }} as a sender address on {{ instance }}.
+This will allow them to send emails that are shown to originate from this email address.
+If that was you, please enter the following confirmation code:
+
+{{ code }}
+
+If this was not requested by you, you can safely ignore this email.
+
+Best regards,
+
+Your {{ instance }} team
+{% endblocktrans %}