Merge commit 'fe2667bb0d3487a32b9da5250402a90482a85fe2' into glitch-soc/merge-upstream

Conflicts:
- `app/lib/content_security_policy.rb`:
  Conflict caused by glitch-soc's support for the extra `EXTRA_DATA_HOSTS`
  environment variable.
  Ported upstream's changes while keeping support for `EXTRA_DATA_HOSTS`.

app/validators/email_mx_validator.rb

@@ -47,6 +47,7 @@ class EmailMxValidator < ActiveModel::Validator
       dns.timeouts = 5
 
       records = dns.getresources(domain, Resolv::DNS::Resource::IN::MX).to_a.map { |e| e.exchange.to_s }
+      next if records == [''] # This domain explicitly rejects emails
 
       ([domain] + records).uniq.each do |hostname|
         ips.concat(dns.getresources(hostname, Resolv::DNS::Resource::IN::A).to_a.map { |e| e.address.to_s })