Encourage non-default instance salt

Problem:

Instance administrators need to set a custom loginSalt before running CryptPad in a production environment. This value is supposedly only rarely changed from the default.

Consequences:

The loginSalt makes it such attackers who want to brute-force common credentials must do so again on each CryptPad instance that they wish to attack. If loginSalt is the default one, then there is no protection against this.

Suggestions:

1. Mention the loginSalt more prominently in the documentation.

2. Write an installation script that initializes this salt to a random value.

Drawbacks: