Prevent login credential appearing in URL

Closes #10221.

src/webui/www/public/login.html

@@ -59,7 +59,7 @@
             <img src="images/skin/qbittorrent-tray.svg" alt="qBittorrent logo" />
         </div>
         <div id="formplace" class="col">
-            <form id="loginform">
+            <form id="loginform" method="post">
                 <div class="row">
                     <label for="username">QBT_TR(Username)QBT_TR[CONTEXT=HttpServer]</label><br />
                     <input type="text" id="username" name="username" autocomplete="username" /></div>