Created default authorization and authentication callbacks

* Created corresponding templates
* Changed how Server configuration works
* Ensured that rauthorization approval requests verify their indieauth parameters
* Wrote first passing test for Server, fixed a variety of small errors along the way

src/Callback/SingleUserPasswordAuthenticationCallback.php

@@ -0,0 +1,43 @@
+<?php declare(strict_types=1);
+
+namespace Taproot\IndieAuth\Callback;
+
+use Exception;
+use Nyholm\Psr7\Response;
+use Psr\Http\Message\ServerRequestInterface;
+
+use function Taproot\IndieAuth\renderTemplate;
+
+class SingleUserPasswordAuthenticationCallback {
+	const PASSWORD_FORM_PARAMETER = 'taproot_indieauth_server_password';
+
+	public string $csrfKey;
+	public string $formTemplate;
+	protected array $user;
+	protected string $hashedPassword;
+
+	public function __construct(array $user, string $hashedPassword, ?string $csrfKey=null, ?string $formTemplate=null) {
+		if (!array_key_exists('me', $user) || !is_string($user['me'])) {
+			throw new Exception('The $user array MUST contain a “me” key, the value which must be the user’s canonical URL as a string.');
+		}
+		$this->user = $user;
+		$this->hashedPassword = $hashedPassword;
+		$this->formTemplate = $formTemplate ?? __DIR__ . '/../templates/single_user_password_authentication_form.html.php';
+		$this->csrfKey = $csrfKey ?? \Taproot\IndieAuth\Server::DEFAULT_CSRF_KEY;
+	}
+
+	public function __invoke(ServerRequestInterface $request, string $formAction) {
+		// If the request is a form submission with a matching password, return the corresponding
+		// user data.
+		if ($request->getMethod() == 'POST' && password_verify($request->getParsedBody()[self::PASSWORD_FORM_PARAMETER] ?? '', $this->hashedPassword)) {
+			return $this->user;
+		}
+
+		// Otherwise, return a response containing the password form.
+		return new Response(200, ['content-type' => 'text/html'], renderTemplate($this->formTemplate, [
+			'formAction' => $formAction,
+			'request' => $request,
+			'csrfFormElement' => '<input type="hidden" name="' . htmlentities($this->csrfKey) . '" value="' . htmlentities($request->getAttribute($this->csrfKey)) . '" />'
+		]));
+	}
+}