Allow more than one email address in the spam report input

[rafaehlers]

As of version 1.4.1, it's not possible to add additional email addresses to the spam report input.

[floatytheastronaut]

FreeScout #76396.
https://support.gravitykit.com/conversation/76396?folder_id=1

[rafaehlers]

https://secure.helpscout.net/conversation/2924932100/63426