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

resources/lang/en/common.php

@@ -42,6 +42,8 @@ return [
     'fullscreen' => 'Fullscreen',
     'favourite' => 'Favourite',
     'unfavourite' => 'Unfavourite',
+    'next' => 'Next',
+    'previous' => 'Previous',
 
     // Sort Options
     'sort_options' => 'Sort Options',