Send authentication request too

[emersion]

For now, you need to provide a session and a cookie. Thus, making an automated program which backups emails is not possible.

Instead, do the authentication request in the app and allow users to provide their username, login password and mailbox password.

[scastiel]

I tried to make it, it was more complicated than I thought, the retro-engineering phase is not as easy as it was for mail fetching.
Feel free to make a PR if you know how to do it ;)

[emersion]

If someone wants to do it, here are some hints:

• The authentication code is here: https://github.com/ProtonMail/WebClient/tree/public/src/app/authentication/services
• The srp.js and passwords.js fiels are libraries for ProtonMail's SRP
• To login, you first have to perform an SRP authentication: https://github.com/ProtonMail/WebClient/blob/public/src/app/authentication/services/auth.js#L366
• Then unlock the private key: https://github.com/ProtonMail/WebClient/blob/public/src/app/authentication/services/auth.js#L508