Redesign the authentication systems

This issue is to enhance the security of authentication within Mailu. It is based on a small brain-storming session me and @kaiyou had.

In order to properly secure and manageable 2fa system, we will need to centralize all web logins to a central point (eg admin). This system will forward an authentication token to the final app (webmail / admin). From this system we can extend to 2fa. Additionally, we want to investigate using a more advanced authentication system for IMAP:

[...] there are a couple technos available, OAuth, SAML, and also the good all redirect-and-post-the-login-form-for-me technique. We have to experiment

• Write a central authentication handler in admin
• Implement remote authentication for Roundcube
• Implement remote authentication for Rainloop
• Advanced auth system for IMAP
• Implement 2fa in the central handler

This issue replaces #67, #264 and #685

The issue names the admin central authentication handler, remote authentication for Roundcube and Rainloop, IMAP authentication, and 2FA. Review the existing admin handler and completed webmail integrations first; the remaining work is to define and implement the advanced IMAP authentication system and central-handler 2FA.