Comments: Updated to show as nested threads

Initial functional implementation, a lot of tweaking and adapting to be
done.

app/Activity/Tools/CommentTree.php

@@ -0,0 +1,102 @@
+<?php
+
+namespace BookStack\Activity\Tools;
+
+use BookStack\Activity\Models\Comment;
+use BookStack\Entities\Models\Page;
+
+class CommentTree
+{
+    /**
+     * The built nested tree structure array.
+     * @var array{comment: Comment, depth: int, children: array}[]
+     */
+    protected array $tree;
+    protected array $comments;
+
+    public function __construct(
+        protected Page $page
+    ) {
+        $this->comments = $this->loadComments();
+        $this->tree = $this->createTree($this->comments);
+    }
+
+    public function enabled(): bool
+    {
+        return !setting('app-disable-comments');
+    }
+
+    public function empty(): bool
+    {
+        return count($this->tree) === 0;
+    }
+
+    public function count(): int
+    {
+        return count($this->comments);
+    }
+
+    public function get(): array
+    {
+        return $this->tree;
+    }
+
+    /**
+     * @param Comment[] $comments
+     */
+    protected function createTree(array $comments): array
+    {
+        $byId = [];
+        foreach ($comments as $comment) {
+            $byId[$comment->local_id] = $comment;
+        }
+
+        $childMap = [];
+        foreach ($comments as $comment) {
+            $parent = $comment->parent_id;
+            if (is_null($parent) || !isset($byId[$parent])) {
+                $parent = 0;
+            }
+
+            if (!isset($childMap[$parent])) {
+                $childMap[$parent] = [];
+            }
+            $childMap[$parent][] = $comment->local_id;
+        }
+
+        $tree = [];
+        foreach ($childMap[0] as $childId) {
+            $tree[] = $this->createTreeForId($childId, 0, $byId, $childMap);
+        }
+
+        return $tree;
+    }
+
+    protected function createTreeForId(int $id, int $depth, array &$byId, array &$childMap): array
+    {
+        $childIds = $childMap[$id] ?? [];
+        $children = [];
+
+        foreach ($childIds as $childId) {
+            $children[] = $this->createTreeForId($childId, $depth + 1, $byId, $childMap);
+        }
+
+        return [
+            'comment' => $byId[$id],
+            'depth' => $depth,
+            'children' => $children,
+        ];
+    }
+
+    protected function loadComments(): array
+    {
+        if (!$this->enabled()) {
+            return [];
+        }
+
+        return $this->page->comments()
+            ->with('createdBy')
+            ->get()
+            ->all();
+    }
+}