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.

spec/lib/guardian_spec.rb

@@ -163,6 +163,16 @@ RSpec.describe Guardian do
       Flag.reset_flag_settings!
     end
 
+    it "return true for illegal if tl0 and allow_tl0_and_anonymous_users_to_flag_illegal_content" do
+      SiteSetting.flag_post_allowed_groups = ""
+      user.trust_level = TrustLevel[0]
+      expect(Guardian.new(user).post_can_act?(post, :illegal)).to be false
+
+      SiteSetting.email_address_to_report_illegal_content = "illegal@example.com"
+      SiteSetting.allow_tl0_and_anonymous_users_to_flag_illegal_content = true
+      expect(Guardian.new(user).post_can_act?(post, :illegal)).to be true
+    end
+
     it "works as expected for silenced users" do
       UserSilencer.silence(user, admin)