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- For detecting an outdated draft, checked the draft created_at time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.

app/Http/Controllers/PageController.php

@@ -4,6 +4,7 @@ namespace BookStack\Http\Controllers;
 
 use BookStack\Actions\View;
 use BookStack\Entities\Models\Page;
+use BookStack\Entities\Models\PageRevision;
 use BookStack\Entities\Repos\PageRepo;
 use BookStack\Entities\Tools\BookContents;
 use BookStack\Entities\Tools\NextPreviousContentLocator;
@@ -258,32 +259,14 @@ class PageController extends Controller
             return $this->jsonError(trans('errors.guests_cannot_save_drafts'), 500);
         }
 
-        // Check for active editing or time conflict
-        $warnings = [];
-        $jsonResponseWarning = '';
-        $editActivity = new PageEditActivity($page);
-        if ($editActivity->hasActiveEditing()) {
-            $warnings[] = $editActivity->activeEditingMessage();
-        }
-        $userDraft = $this->pageRepo->getUserDraft($page);
-        if ($userDraft !== null) {
-            if ($editActivity->hasPageBeenUpdatedSinceDraftSaved($userDraft)) {
-                $warnings[] = $editActivity->getEditingActiveDraftMessage($userDraft);
-            }
-        }
-        if (count($warnings) > 0) {
-            $jsonResponseWarning = implode("\n", $warnings);
-        }
-
         $draft = $this->pageRepo->updatePageDraft($page, $request->only(['name', 'html', 'markdown']));
-
-        $updateTime = $draft->updated_at->timestamp;
+        $warnings = (new PageEditActivity($page))->getWarningMessagesForDraft($draft);
 
         return response()->json([
             'status'    => 'success',
             'message'   => trans('entities.pages_edit_draft_save_at'),
-            'warning'   => $jsonResponseWarning,
-            'timestamp' => $updateTime,
+            'warning'   => implode("\n", $warnings),
+            'timestamp' => $draft->updated_at->timestamp,
         ]);
     }