auth: enable captcha for sign-up

[mvidalgarcia]

Original request: https://github.com/reanahub/reana/issues/469#issuecomment-746448365

This can be a Helm option to enable it on-demand.

Alternatives:

• Google reCAPTCHA: https://developers.google.com/recaptcha
  • Invenio uses it
• Curated list of captcha libs: https://zyszys.github.io/awesome-captcha/#libraries
• Our custom checkbox
• ...