Reviewed and refactored next/previous navigation button implementation

- Updated styling to include item name.
- Extracted used text to translations.
- Updated the design to better suit the surrounding blocks.
- Removed newly added model/repo methods.
- Moved core logic out of controller and instead into a "NextPreviousContentLocator"
helper with re-uses the output from the book-tree generation.
- Also added the system to chapters.

For #2511

app/Entities/Repos/PageRepo.php

@@ -468,10 +468,4 @@ class PageRepo
             ->where('page_id', '=', $page->id)
             ->orderBy('created_at', 'desc');
     }
-    /**
-     * Get page details by chapter ID.
-     */
-    public function getPageByChapterID(int $id){
-        return Page::visible()->where('chapter_id', '=', $id)->get(['id','slug']);
-    }
 }