Migrate cache purge function to a proper class

And update dependencies and tests.

Note that SESSION['tags'] has been removed a log ago

application/render/PageCacheManager.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Shaarli\Render;
+
+/**
+ * Cache utilities
+ */
+class PageCacheManager
+{
+    /** @var string Cache directory */
+    protected $pageCacheDir;
+
+    public function __construct(string $pageCacheDir)
+    {
+        $this->pageCacheDir = $pageCacheDir;
+    }
+
+    /**
+     * Purges all cached pages
+     *
+     * @return string|null an error string if the directory is missing
+     */
+    public function purgeCachedPages(): ?string
+    {
+        if (!is_dir($this->pageCacheDir)) {
+            $error = sprintf(t('Cannot purge %s: no directory'), $this->pageCacheDir);
+            error_log($error);
+
+            return $error;
+        }
+
+        array_map('unlink', glob($this->pageCacheDir . '/*.cache'));
+
+        return null;
+    }
+
+    /**
+     * Invalidates caches when the database is changed or the user logs out.
+     */
+    public function invalidateCaches(): void
+    {
+        // Purge page cache shared by sessions.
+        $this->purgeCachedPages();
+    }
+}