FEATURE: implements drafts for threads (#24483)

This commit implements drafts for threads by adding a new `thread_id` column to `chat_drafts` table. This column is used to create draft keys on the frontend which are a compound key of the channel and the thread. If the draft is only for the channel, the key will be `c-${channelId}`, if for a thread: `c-${channelId}:t-${threadId}`.

This commit also moves the draft holder from the service to the channel or thread model. The current draft can now always be accessed by doing: `channel.draft` or `thread.draft`.

Other notable changes of this commit:
- moves ChatChannel to gjs
- moves ChatThread to gjs

plugins/chat/test/javascripts/helpers/chat-default-pretender.js

@@ -0,0 +1,8 @@
+export default function applyDefaultHandlers(helpers) {
+  this.post("/chat/api/channels/:channel_id/drafts", () =>
+    helpers.response({})
+  );
+  this.post("/chat/api/channels/:channel_id/threads/:thread_id/drafts", () =>
+    helpers.response({})
+  );
+}

plugins/chat/test/javascripts/unit/services/chat-drafts-manager-test.js

@@ -12,36 +12,24 @@ module(
       this.subject = getOwner(this).lookup("service:chat-drafts-manager");
     });
 
-    hooks.afterEach(function () {
-      this.subject.reset();
-    });
-
-    test("storing and retrieving message", function (assert) {
+    test("storing and retrieving message", async function (assert) {
       const message1 = fabricators.message();
-      this.subject.add(message1);
 
-      assert.strictEqual(
-        this.subject.get({ channelId: message1.channel.id }),
-        message1
-      );
+      await this.subject.add(message1, message1.channel.id);
+
+      assert.strictEqual(this.subject.get(message1.channel.id), message1);
 
       const message2 = fabricators.message();
-      this.subject.add(message2);
 
-      assert.strictEqual(
-        this.subject.get({ channelId: message2.channel.id }),
-        message2
-      );
+      await this.subject.add(message2, message2.channel.id);
+
+      assert.strictEqual(this.subject.get(message2.channel.id), message2);
     });
 
-    test("stores only chat messages", function (assert) {
-      assert.throws(function () {
-        this.subject.add({ foo: "bar" });
-      }, /instance of ChatMessage/);
-    });
+    test("#reset", async function (assert) {
+      const message = fabricators.message();
 
-    test("#reset", function (assert) {
-      this.subject.add(fabricators.message());
+      await this.subject.add(message, message.channel.id);
 
       assert.strictEqual(Object.keys(this.subject.drafts).length, 1);