fix session_store_active functionality

At the moment and since 1.7.19, the `system.languages.session_store_active`
setting has no effect.

Session must be initialized before URI for `$language->setActiveFromUri($uri)`
(called from `$this->initializeUri($config) -> $uri->init()`) to
properly retrieve / store `active_language` in Session.

This was previously detected in #3269 as per the code comment, but got
reversed in 2e9fe80e33.

system/src/Grav/Common/Processors/InitializeProcessor.php

@@ -105,12 +105,12 @@ class InitializeProcessor extends ProcessorBase
         // TODO: remove in 2.0.
         $this->container['accounts'];
 
+        // Initialize session (used by URI, see issue #3269).
+        $this->initializeSession($config);
+
         // Initialize URI (uses session, see issue #3269).
         $this->initializeUri($config);
 
-        // Initialize session.
-        $this->initializeSession($config);
-
         // Grav may return redirect response right away.
         $redirectCode = (int)$config->get('system.pages.redirect_trailing_slash', 1);
         if ($redirectCode) {