Let the user pick 2FA method

Users should be able to choose the two-factor auth mechanism they prefer, when multiple ones are available.

server/templates/login_otp.html

@@ -18,4 +18,12 @@
 
     </form>
 
+    {{if .AuthResponse.Has2FAMethod "u2f"}}
+<p>
+  <a href="{{.URLPrefix}}/login?2fa=u2f">
+    Use a hardware token instead.
+  </a>
+</p>
+    {{end}}
+
 {{template "footer" .}}