add register, login and main page with basic functionality

app/templates/login.html

@@ -0,0 +1,9 @@
+<h1>Login</h1>
+
+<form method="POST">
+  <input name="username" placeholder="Username" required />
+  <input name="password" type="password" placeholder="Password" required />
+  <button type="submit">Login</button>
+</form>
+
+<a href="{{ url_for('auth.register') }}">Register</a>