Fix reverse proxy subpath support (#192)

templates/pages/create.html

@@ -8,7 +8,7 @@
 
     </header>
     <main class="mt-4">
-        <form id="create" class="space-y-4" method="post" action="/">
+        <form id="create" class="space-y-4" method="post" action="{{ $.c.ExternalUrl }}/">
             <div>
                 <p class="cursor-pointer select-none" id="gist-metadata-btn">Metadata ▼</p>
                 <div class="grid grid-cols-12 gap-x-4 mt-1 hidden" id="gist-metadata">