Add recaptcha to login modal

[kaitlinnewson]

Describe the bug

When a user clicks 'login', a modal opens, but the recaptcha is missing. Users can still login once redirected to the login page, but we should add the recaptcha to the modal as well.

Applies to 3.4+ when captcha on login was added.

To Reproduce

Steps to reproduce the behavior:

1. Go to the home page of a journal with the theme enabled while logged out, and recaptcha setup in the config and captcha_on_login = on.
2. Click login.
3. Try to login, noting the missing recaptcha.
4. Get an error and taken to the login page, where the recaptcha appears.

Expected behavior

The recaptcha should appear on the login modal.

Screenshots

Login modal with missing recaptcha:

Login page with recaptcha: