Add cURL support to HTTP Client

- Add HTTP_PROXY_EXCLUDE option when cURL is used
- Show HTTP client backend in about page
- Fallback to legacy Stream Contexts if cURL extension is not available

app/Template/config/about.php

@@ -35,6 +35,10 @@
             <?= t('PHP SAPI:') ?>
             <strong><?= PHP_SAPI ?></strong>
         </li>
+        <li>
+            <?= t('HTTP Client:') ?>
+            <strong><?= Kanboard\Core\Http\Client::backend() ?></strong>
+        </li>
         <li>
             <?= t('OS version:') ?>
             <strong><?= @php_uname('s').' '.@php_uname('r') ?></strong>