FIX: correctly handle subscriptions (#24270)

Subscriptions manager have been a pain since the beginning, one of the problem is that thread and channels behave mostly the same but with various small difference which I expect to increase over time.

Trying to use subclasses for this case has proven to be a mistake, this commit now uses a class for each case (channel, thread) which for now contains a lot of duplication, which might be reduced in the future but has the merit to make reasoning about each case very simple.

This refactor is fixing a bug introduced in 90efdd7f9d which was causing the wrong channel to be unsubscribed, this shouldn't be possible anymore. We had tests for this which were disabled due to flakeyness, I will consider re-enabling them in the future.

Other notes:
- notices had been added to the subscriptions manager service, they have been moved into their own dedicated service: `ChatChannelNoticesManager`
- the `(each model)` trick used in `<ChatChannel />` since 90efdd7f9d to ensure atomicity has been applied to `<ChatThread />` too

plugins/chat/test/javascripts/components/chat-notices-test.js

@@ -12,9 +12,7 @@ module("Discourse Chat | Component | chat-notice", function (hooks) {
 
   test("displays all notices for a channel", async function (assert) {
     this.channel = fabricators.channel();
-    this.manager = this.container.lookup(
-      "service:chatChannelPaneSubscriptionsManager"
-    );
+    this.manager = this.container.lookup("service:chatChannelNoticesManager");
     this.manager.handleNotice({
       channel_id: this.channel.id,
       text_content: "hello",
@@ -40,9 +38,7 @@ module("Discourse Chat | Component | chat-notice", function (hooks) {
 
   test("Notices can be cleared", async function (assert) {
     this.channel = fabricators.channel();
-    this.manager = this.container.lookup(
-      "service:chatChannelPaneSubscriptionsManager"
-    );
+    this.manager = this.container.lookup("service:chatChannelNoticesManager");
     this.manager.handleNotice({
       channel_id: this.channel.id,
       text_content: "hello",
@@ -66,9 +62,7 @@ module("Discourse Chat | Component | chat-notice", function (hooks) {
   });
   test("MentionWithoutMembership notice renders", async function (assert) {
     this.channel = fabricators.channel();
-    this.manager = this.container.lookup(
-      "service:chatChannelPaneSubscriptionsManager"
-    );
+    this.manager = this.container.lookup("service:chatChannelNoticesManager");
     const text = "Joffrey can't chat, hermano";
     this.manager.handleNotice({
       channel_id: this.channel.id,