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/Tools/NextPreviousContentLocator.php

@@ -0,0 +1,69 @@
+<?php namespace BookStack\Entities\Tools;
+
+use BookStack\Entities\Models\BookChild;
+use BookStack\Entities\Models\Entity;
+use Illuminate\Support\Collection;
+
+/**
+ * Finds the next or previous content of a book element (page or chapter).
+ */
+class NextPreviousContentLocator
+{
+    protected $relativeBookItem;
+    protected $flatTree;
+    protected $currentIndex = null;
+
+    /**
+     * NextPreviousContentLocator constructor.
+     */
+    public function __construct(BookChild $relativeBookItem, Collection $bookTree)
+    {
+        $this->relativeBookItem = $relativeBookItem;
+        $this->flatTree = $this->treeToFlatOrderedCollection($bookTree);
+        $this->currentIndex = $this->getCurrentIndex();
+    }
+
+    /**
+     * Get the next logical entity within the book hierarchy.
+     */
+    public function getNext(): ?Entity
+    {
+        return $this->flatTree->get($this->currentIndex + 1);
+    }
+
+    /**
+     * Get the next logical entity within the book hierarchy.
+     */
+    public function getPrevious(): ?Entity
+    {
+        return $this->flatTree->get($this->currentIndex - 1);
+    }
+
+    /**
+     * Get the index of the current relative item.
+     */
+    protected function getCurrentIndex(): ?int
+    {
+        $index = $this->flatTree->search(function (Entity $entity) {
+            return get_class($entity) === get_class($this->relativeBookItem)
+                && $entity->id === $this->relativeBookItem->id;
+        });
+        return $index === false ? null : $index;
+    }
+
+    /**
+     * Convert a book tree collection to a flattened version
+     * where all items follow the expected order of user flow.
+     */
+    protected function treeToFlatOrderedCollection(Collection $bookTree): Collection
+    {
+        $flatOrdered = collect();
+        /** @var Entity $item */
+        foreach ($bookTree->all() as $item) {
+            $flatOrdered->push($item);
+            $childPages = $item->visible_pages ?? [];
+            $flatOrdered = $flatOrdered->concat($childPages);
+        }
+        return $flatOrdered;
+    }
+}