In recent Odoo versions, the login field is tightly linked to the email field
(the field is displayed with an "Email" label, and it will populate the email of the user if it contains an "@" character).

Yet for the users who uses a distinct login and email, this is confusing.

The goal of this module is to untangle (a bit) these two fields. Here are the changes:

* In the `res.users` form view:
    * Display a "Login" label instead of "Email" on the `login` field
    * Show the currently invisible `email` field and its label
* In the `res.users` tree view:
    * Display the `email` field next to the `login` field
* Change the login layout to prompt for a "Login" instead of an "Email" (compatible with the web_enterprise layout)
