FEATURE: Export chat messages to CSV file (#22113)

To export chat messages, go to `/admin/plugins/chat` and click the Create export 
button in the _Export chat messages_ section. You'll receive a direct message 
when the export is finished.

Currently, this exports all messages from the last 6 months, but not more than 
10000 messages.

This exports all chat messages, including messages from private channels and 
users' direct conversations. This also exports messages that were deleted.

plugins/chat/assets/javascripts/discourse/components/chat/admin/export-messages.hbs

@@ -0,0 +1,10 @@
+<section class="admin-section">
+  <h3>{{i18n "chat.admin.export_messages.title"}}</h3>
+  <p>{{i18n "chat.admin.export_messages.description"}}</p>
+  <DButton
+    @label="chat.admin.export_messages.create_export"
+    @title="chat.admin.export_messages.create_export"
+    @class="btn-primary"
+    @action={{action this.exportMessages}}
+  />
+</section>

plugins/chat/assets/javascripts/discourse/components/chat/admin/export-messages.js

@@ -0,0 +1,22 @@
+import Component from "@ember/component";
+import { action } from "@ember/object";
+import { inject as service } from "@ember/service";
+import { popupAjaxError } from "discourse/lib/ajax-error";
+import I18n from "I18n";
+
+export default class ExportMessages extends Component {
+  @service chatAdminApi;
+  @service dialog;
+
+  @action
+  async exportMessages() {
+    try {
+      await this.chatAdminApi.exportMessages();
+      this.dialog.alert(
+        I18n.t("chat.admin.export_messages.export_has_started")
+      );
+    } catch (error) {
+      popupAjaxError(error);
+    }
+  }
+}

plugins/chat/assets/javascripts/discourse/services/chat-admin-api.js

@@ -0,0 +1,19 @@
+import Service from "@ember/service";
+import { ajax } from "discourse/lib/ajax";
+
+export default class ChatAdminApi extends Service {
+  async exportMessages() {
+    await this.#post(`/export/messages`);
+  }
+
+  get #basePath() {
+    return "/chat/admin";
+  }
+
+  #post(endpoint, data = {}) {
+    return ajax(`${this.#basePath}${endpoint}`, {
+      type: "POST",
+      data,
+    });
+  }
+}

plugins/chat/assets/javascripts/discourse/templates/admin-plugins-chat.hbs

@@ -1,3 +1,5 @@
+<Chat::Admin::ExportMessages />
+
 {{#if this.selectedWebhook}}
   <DButton
     @class="incoming-chat-webhooks-back"