DEV: Rename direct message related models

This is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.

For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.

plugins/chat/test/javascripts/components/chat-channel-leave-btn-test.js

@@ -19,7 +19,7 @@ module("Discourse Chat | Component | chat-channel-leave-btn", function (hooks) {
       this.set("onLeaveChannel", () => this.set("foo", 2));
       this.set("channel", {
         id: 1,
-        chatable_type: "DirectMessageChannel",
+        chatable_type: "DirectMessage",
         chatable: {
           users: [{ id: 1 }],
         },
@@ -42,7 +42,7 @@ module("Discourse Chat | Component | chat-channel-leave-btn", function (hooks) {
     template: hbs`{{chat-channel-leave-btn channel=channel}}`,
 
     beforeEach() {
-      this.set("channel", { chatable_type: "DirectMessageChannel" });
+      this.set("channel", { chatable_type: "DirectMessage" });
     },
 
     async test(assert) {

plugins/chat/test/javascripts/components/chat-composer-placeholder-test.js

@@ -20,7 +20,7 @@ module(
         this.set(
           "chatChannel",
           ChatChannel.create({
-            chatable_type: "DirectMessageChannel",
+            chatable_type: "DirectMessage",
             chatable: {
               users: [{ id: 1 }],
             },
@@ -43,7 +43,7 @@ module(
         this.set(
           "chatChannel",
           ChatChannel.create({
-            chatable_type: "DirectMessageChannel",
+            chatable_type: "DirectMessage",
             chatable: {
               users: [
                 { name: "Tomtom" },

plugins/chat/test/javascripts/components/chat-retention-reminder-test.js

@@ -59,7 +59,7 @@ module(
       async beforeEach() {
         this.set(
           "chatChannel",
-          ChatChannel.create({ chatable_type: "DirectMessageChannel" })
+          ChatChannel.create({ chatable_type: "DirectMessage" })
         );
         set(this.currentUser, "needs_dm_retention_reminder", true);
         this.siteSettings.chat_dm_retention_days = 100;
@@ -79,7 +79,7 @@ module(
       async beforeEach() {
         this.set(
           "chatChannel",
-          ChatChannel.create({ chatable_type: "DirectMessageChannel" })
+          ChatChannel.create({ chatable_type: "DirectMessage" })
         );
         set(this.currentUser, "needs_dm_retention_reminder", false);
         this.siteSettings.chat_dm_retention_days = 100;