FEATURE: setting allowing tl0/anonymous flag illegal content (#30785)

The new site setting `allow_anonymous_and_tl0_to_flag_illegal` allows
tl0 users to flag illegal content. In addition, anonymous users are
instructed on how to flag illegal content by sending emails.

Also `email_address_to_report_illegal_content` setting is added. If not
provided, then the site contact email is used.

lib/guardian/post_guardian.rb

@@ -94,6 +94,10 @@ module PostGuardian
                 post.topic.private_message?
             )
         ) ||
+          (
+            action_key == :illegal &&
+              SiteSetting.allow_tl0_and_anonymous_users_to_flag_illegal_content
+          ) ||
           # not a flagging action, and haven't done it already
           not(is_flag || already_taken_this_action) &&
             # nothing except flagging on archived topics