DEV: Make admin-start-backup template extendable (#18904)

We need to change the content of this modal in different
plugins based on context, so move some parts of it into
the JS file.

app/assets/javascripts/admin/addon/controllers/modals/admin-start-backup.js

@@ -1,9 +1,22 @@
 import Controller, { inject as controller } from "@ember/controller";
+import discourseComputed from "discourse-common/utils/decorators";
 import ModalFunctionality from "discourse/mixins/modal-functionality";
 
 export default Controller.extend(ModalFunctionality, {
   adminBackupsLogs: controller(),
 
+  @discourseComputed
+  warningMessage() {
+    // this is never shown here, but we may want to show different
+    // messages in plugins
+    return "";
+  },
+
+  @discourseComputed
+  yesLabel() {
+    return "yes_value";
+  },
+
   actions: {
     startBackupWithUploads() {
       this.send("closeModal");

app/assets/javascripts/admin/addon/templates/modal/admin-start-backup.hbs

@@ -1,5 +1,8 @@
 <DModalBody @title="admin.backups.operations.backup.confirm">
-  <DButton @class="btn-primary backup-with-uploads" @action={{action "startBackupWithUploads"}} @label="yes_value" />
+  {{#if this.warningMessage}}
+    <div class="alert alert-warning">{{html-safe this.warningMessage}}</div>
+  {{/if}}
+  <DButton @class="btn-primary backup-with-uploads" @action={{action "startBackupWithUploads"}} @label={{this.yesLabel}} />
   <DButton @class="backup-no-uploads" @action={{action "startBackupWithoutUploads"}} @label="admin.backups.operations.backup.without_uploads" />
   <DButton @class="btn-default" @action={{action "cancel"}} @label="no_value" />
 </DModalBody>