Sorting: Renamed sort set to sort rule

Renamed based on feedback from Tim and Script on Discord.
Also fixed flaky test

app/Sorting/SortRule.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace BookStack\Sorting;
+
+use BookStack\Activity\Models\Loggable;
+use BookStack\Entities\Models\Book;
+use Carbon\Carbon;
+use Illuminate\Database\Eloquent\Collection;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+/**
+ * @property int $id
+ * @property string $name
+ * @property string $sequence
+ * @property Carbon $created_at
+ * @property Carbon $updated_at
+ */
+class SortRule extends Model implements Loggable
+{
+    use HasFactory;
+
+    /**
+     * @return SortRuleOperation[]
+     */
+    public function getOperations(): array
+    {
+        return SortRuleOperation::fromSequence($this->sequence);
+    }
+
+    /**
+     * @param SortRuleOperation[] $options
+     */
+    public function setOperations(array $options): void
+    {
+        $values = array_map(fn (SortRuleOperation $opt) => $opt->value, $options);
+        $this->sequence = implode(',', $values);
+    }
+
+    public function logDescriptor(): string
+    {
+        return "({$this->id}) {$this->name}";
+    }
+
+    public function getUrl(): string
+    {
+        return url("/settings/sorting/rules/{$this->id}");
+    }
+
+    public function books(): HasMany
+    {
+        return $this->hasMany(Book::class);
+    }
+
+    public static function allByName(): Collection
+    {
+        return static::query()
+            ->withCount('books')
+            ->orderBy('name', 'asc')
+            ->get();
+    }
+}