FEATURE: Chat side panel with threads initial skeleton (#20209)

This commit introduces the skeleton of the chat thread UI. The
structure of the components looks like this. Its done this way
so the side panel can be used for other things as well if we wish,
not just for threads:

```
.main-chat-outlet
   <ChatLivePane />
   <ChatSidePanel>
     <-- rendered with {{outlet}} -->
     <ChatThread />
   </ChatSidePanel>
```

Later on the `ChatThreadList` will be rendered here as well.
Now, when you go to a channel you can open a thread by clicking
on either the Open Thread message action button or by clicking on
the reply indicator. This will take you to a route like `chat/c/:slug/:channelId/t/:threadId`.
This works on mobile as well.

This commit includes basic serializers and routes for threads,
as well as a new `ChatThreadsManager` service in JS that caches
threads for a channel the same way the channel threads manager does.

The chat messages inside the thread are intentionally left out
until a later PR.

**NOTE: These changes are gated behind the site setting enable_experimental_chat_threaded_discussions
and the threading_enabled boolean on a ChatChannel**

plugins/chat/spec/requests/api/chat_channel_threads_controller_spec.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe Chat::Api::ChatChannelThreadsController do
+  fab!(:current_user) { Fabricate(:user) }
+
+  before do
+    SiteSetting.chat_enabled = true
+    SiteSetting.chat_allowed_groups = Group::AUTO_GROUPS[:everyone]
+    SiteSetting.enable_experimental_chat_threaded_discussions = true
+    Group.refresh_automatic_groups!
+    sign_in(current_user)
+  end
+
+  describe "show" do
+    context "when thread does not exist" do
+      fab!(:thread) { Fabricate(:chat_thread, original_message: Fabricate(:chat_message)) }
+
+      it "returns 404" do
+        thread.destroy!
+        get "/chat/api/channels/#{thread.channel_id}/threads/#{thread.id}"
+        expect(response.status).to eq(404)
+      end
+    end
+
+    context "when thread exists" do
+      fab!(:thread) { Fabricate(:chat_thread, original_message: Fabricate(:chat_message)) }
+
+      it "works" do
+        get "/chat/api/channels/#{thread.channel_id}/threads/#{thread.id}"
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["thread"]["id"]).to eq(thread.id)
+      end
+
+      context "when the channel_id does not match the thread id" do
+        fab!(:other_channel) { Fabricate(:chat_channel) }
+
+        it "returns 404" do
+          get "/chat/api/channels/#{other_channel.id}/threads/#{thread.id}"
+          expect(response.status).to eq(404)
+        end
+      end
+
+      context "when enable_experimental_chat_threaded_discussions is disabled" do
+        before { SiteSetting.enable_experimental_chat_threaded_discussions = false }
+
+        it "returns 404" do
+          get "/chat/api/channels/#{thread.channel_id}/threads/#{thread.id}"
+          expect(response.status).to eq(404)
+        end
+      end
+
+      context "when user cannot access the channel" do
+        before do
+          thread.channel.update!(chatable: Fabricate(:private_category, group: Fabricate(:group)))
+        end
+
+        it "returns 403" do
+          get "/chat/api/channels/#{thread.channel_id}/threads/#{thread.id}"
+          expect(response.status).to eq(403)
+        end
+      end
+
+      context "when user cannot chat" do
+        before { SiteSetting.chat_allowed_groups = Group::AUTO_GROUPS[:trust_level_4] }
+
+        it "returns 403" do
+          get "/chat/api/channels/#{thread.channel_id}/threads/#{thread.id}"
+          expect(response.status).to eq(403)
+        end
+      end
+    end
+  end
+end