mod_http_oauth2: Add support for the OpenID 'login_hint' parameter
This allows the client to suggest to the authorization screen which user
is trying to login, so they don't have to fill that in twice if they
already did so at the client.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>{site_name} - Sign in</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<main>
<h1>{site_name}</h1>
<fieldset>
<legend>Sign in</legend>
<p>Sign in to your account to continue.</p>
{state.error&<div class="error">
<p>{state.error}</p>
</div>}
<form method="post">
<input type="text" name="username" placeholder="Username" aria-label="Username" required {extra.no_username_hint&autofocus}{extra.username_hint& value="{extra.username_hint?}"}><br/>
<input type="password" name="password" placeholder="Password" aria-label="Password" autocomplete="current-password" required {extra.username_hint&autofocus}><br/>
<input type="submit" value="Sign in">
</form>
</fieldset>
</main>
</body>
</html>