check php-extension requirements not only on installation (e.g. when php version was changed)

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

templates/Froxlor/misc/missingextensionhint.html.twig

@@ -0,0 +1,18 @@
+{% extends "Froxlor/base.html.twig" %}
+
+{% block content %}
+<div class="container my-auto">
+<div class="alert alert-warning fade show" role="alert">
+	<h4 class="alert-heading">
+		Whoops!
+	</h4>
+	<p>It looks like you are missing some of the required php requirements.</p>
+	<p>Please ensure that the following extension for the currently used php version ({{ phpversion }}) are installed:</p>
+	<pre>{{ missing_extensions }}</pre>
+	<hr>
+	<p class="mt-1 text-center">
+		<a href="" class="btn btn-primary" title="Reload page">Reload</a>
+	</p>
+</div>
+</div>
+{% endblock %}