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({})
+  );
+}