Strengthen validation of URL in proxy services

This should definitively rule out any circumstances, where invalid URLs could cause problems.

Both URL validity is checked before it is forwarded to the URL shortener proxy _and_ the host part is explicitly compared to make sure the domain is really the same one.

TOOD:
* [ ] some tests may be needed here (hmpff…)

lib/Proxy/AbstractProxy.php

@@ -49,7 +49,14 @@ abstract class AbstractProxy
      */
     public function __construct(Configuration $conf, string $link)
     {
-        if (!str_starts_with($link, $conf->getKey('basepath') . '?')) {
+        if (!filter_var($link, FILTER_VALIDATE_URL, FILTER_FLAG_PATH_REQUIRED & FILTER_FLAG_QUERY_REQUIRED)) {
+            $this->_error = 'Invalid URL given.';
+            return;
+        }
+        
+        if (!str_starts_with($link, $conf->getKey('basepath') . '?') ||
+            parse_url($link, PHP_URL_HOST) != parse_url($conf->getKey('basepath'), PHP_URL_HOST)
+           ) {
             $this->_error = 'Trying to shorten a URL that isn\'t pointing at our instance.';
             return;
         }