Reviewed and refactored additional editor draft save warnings

- Added testing to cover warning cases.
- Refactored logic to be simpler and move much of the business out of
  the controller.
- Added new message that's more suitable to the case this was handling.
- For detecting an outdated draft, checked the draft created_at time
  instead of updated_at to better fit the scenario being checked.
- Updated some method types to align with those potentially being used
  in the logic of the code.
- Added a cache of shown messages on the front-end to prevent them
  re-showing on every save during the session, even if dismissed.

tests/Entity/PageDraftTest.php

@@ -4,6 +4,7 @@ namespace Tests\Entity;
 
 use BookStack\Entities\Models\Book;
 use BookStack\Entities\Models\Page;
+use BookStack\Entities\Models\PageRevision;
 use BookStack\Entities\Repos\PageRepo;
 use Tests\TestCase;
 
@@ -80,6 +81,65 @@ class PageDraftTest extends TestCase
             ->assertElementNotContains('.notification', 'Admin has started editing this page');
     }
 
+    public function test_draft_save_shows_alert_if_draft_older_than_last_page_update()
+    {
+        $admin = $this->getAdmin();
+        $editor = $this->getEditor();
+        /** @var Page $page */
+        $page = Page::query()->first();
+
+        $this->actingAs($editor)->put('/ajax/page/' . $page->id . '/save-draft', [
+            'name' => $page->name,
+            'html' => '<p>updated draft</p>',
+        ]);
+
+        /** @var PageRevision $draft */
+        $draft = $page->allRevisions()
+            ->where('type', '=', 'update_draft')
+            ->where('created_by', '=', $editor->id)
+            ->first();
+        $draft->created_at = now()->subMinute(1);
+        $draft->save();
+
+        $this->actingAs($admin)->put($page->refresh()->getUrl(), [
+            'name' => $page->name,
+            'html' => '<p>admin update</p>',
+        ]);
+
+        $resp = $this->actingAs($editor)->put('/ajax/page/' . $page->id . '/save-draft', [
+            'name' => $page->name,
+            'html' => '<p>updated draft again</p>',
+        ]);
+
+        $resp->assertJson([
+            'warning' => 'This page has been updated since this draft was created. It is recommended that you discard this draft or take care not to overwrite any page changes.',
+        ]);
+    }
+
+    public function test_draft_save_shows_alert_if_draft_edit_started_by_someone_else()
+    {
+        $admin = $this->getAdmin();
+        $editor = $this->getEditor();
+        /** @var Page $page */
+        $page = Page::query()->first();
+
+        $this->actingAs($admin)->put('/ajax/page/' . $page->id . '/save-draft', [
+            'name' => $page->name,
+            'html' => '<p>updated draft</p>',
+        ]);
+
+        $resp = $this->actingAs($editor)->put('/ajax/page/' . $page->id . '/save-draft', [
+            'name' => $page->name,
+            'html' => '<p>updated draft again</p>',
+        ]);
+
+        $resp->assertJson([
+            'warning' => 'Admin has started editing this page in the last 60 minutes. Take care not to overwrite each other\'s updates!',
+        ]);
+    }
+
+
+
     public function test_draft_pages_show_on_homepage()
     {
         /** @var Book $book */