Added reference storage system, and command to re-index

Also re-named/orgranized some files for this, to make them "References"
specific instead of a subset of "Util".

app/References/ReferenceService.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace BookStack\References;
+
+use BookStack\Entities\Models\Page;
+use Illuminate\Database\Eloquent\Collection;
+
+class ReferenceService
+{
+
+    /**
+     * Update the outgoing references for the given page.
+     */
+    public function updateForPage(Page $page): void
+    {
+        $this->updateForPages([$page]);
+    }
+
+    /**
+     * Update the outgoing references for all pages in the system.
+     */
+    public function updateForAllPages(): void
+    {
+        Reference::query()
+            ->where('from_type', '=', (new Page())->getMorphClass())
+            ->truncate();
+
+        Page::query()->select(['id', 'html'])->chunk(100, function(Collection $pages) {
+            $this->updateForPages($pages->all());
+        });
+    }
+
+    /**
+     * Update the outgoing references for the pages in the given array.
+     *
+     * @param Page[] $pages
+     */
+    protected function updateForPages(array $pages): void
+    {
+        if (count($pages) === 0) {
+            return;
+        }
+
+        $parser = CrossLinkParser::createWithEntityResolvers();
+        $references = [];
+
+        $pageIds = array_map(fn(Page $page) => $page->id, $pages);
+        Reference::query()
+            ->where('from_type', '=', $pages[0]->getMorphClass())
+            ->whereIn('from_id', $pageIds)
+            ->delete();
+
+        foreach ($pages as $page) {
+            $models = $parser->extractLinkedModels($page->html);
+
+            foreach ($models as $model) {
+                $references[] = [
+                    'from_id' => $page->id,
+                    'from_type' => $page->getMorphClass(),
+                    'to_id' => $model->id,
+                    'to_type' => $model->getMorphClass(),
+                ];
+            }
+        }
+
+        foreach (array_chunk($references, 1000) as $referenceDataChunk) {
+            Reference::query()->insert($referenceDataChunk);
+        }
+    }
+
+}