FIX: IMAP allow unknown senders to reply to group topics via email (#11877)

Adds a new column/setting to groups, allow_unknown_sender_topic_replies, which is default false. When enabled, this scenario is allowed via IMAP:

* OP sends an email to the support email address which is synced to a group inbox via IMAP, creating a group topic
* Group user replies to the group topic
* An email notification is sent to the OP of the topic via GroupSMTPMailer
* The OP has several email accounts and the reply is sent to all of them, or they forward their reply to another email account
* The OP replies from a different email address than the OP (gloria@gmail.com instead of gloria@hey.com for example)
* The a new staged user is created, the new reply is accepted and added to the topic, and the staged user is added to the topic allowed users

Without allow_unknown_sender_topic_replies enabled the new reply creates an entirely new topic (because the email address it is sent from is not previously part of the topic email chain).

spec/components/email/receiver_spec.rb

@@ -951,8 +951,9 @@ describe Email::Receiver do
         expect { process(:email_reply_2) }.to change { topic.posts.count }.by(1).and change { Topic.count }.by(0)
       end
 
-      it "creates a new topic when the sender is not known" do
+      it "creates a new topic when the sender is not known and the group does not allow unknown senders to reply to topics" do
         IncomingEmail.where(message_id: '34@foo.bar.mail').update(cc_addresses: 'three@foo.com')
+        group.update(allow_unknown_sender_topic_replies: false)
         expect { process(:email_reply_2) }.to change { topic.posts.count }.by(0).and change { Topic.count }.by(1)
       end
 
@@ -960,6 +961,25 @@ describe Email::Receiver do
         IncomingEmail.where(message_id: '34@foo.bar.mail').update(message_id: '99@foo.bar.mail')
         expect { process(:email_reply_2) }.to change { topic.posts.count }.by(0).and change { Topic.count }.by(1)
       end
+
+      it "includes the sender on the topic when the message id is known, the sender is not known, and the group allows unknown senders to reply to topics" do
+        IncomingEmail.where(message_id: '34@foo.bar.mail').update(cc_addresses: 'three@foo.com')
+        group.update(allow_unknown_sender_topic_replies: true)
+        expect { process(:email_reply_2) }.to change { topic.posts.count }.by(1).and change { Topic.count }.by(0)
+      end
+
+      context "when the sender is not in the topic allowed users" do
+        before do
+          user = User.find_by_email("two@foo.com")
+          topic.topic_allowed_users.find_by(user: user).destroy
+        end
+
+        it "adds them to the topic at the same time" do
+          IncomingEmail.where(message_id: '34@foo.bar.mail').update(cc_addresses: 'three@foo.com')
+          group.update(allow_unknown_sender_topic_replies: true)
+          expect { process(:email_reply_2) }.to change { topic.posts.count }.by(1).and change { Topic.count }.by(0)
+        end
+      end
     end
   end