users: Add email address field during first boot

- Mostly because administrators won't discover the email address field for the
user later on.

- This field is important to be able to run 'reset password' operation.

Tests:

- In stable and testing containers, run first boot wizard. Enter the email
address during the first boot and see that it was saved as part of user account.
Leave the email address blank and it is possible to proceed. User account show
blank email address.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/users/forms.py

@@ -382,11 +382,16 @@ class UserChangePasswordForm(PasswordConfirmForm, SetPasswordForm):
         return user
 
 
-class FirstBootForm(ValidNewUsernameCheckMixin, auth.forms.UserCreationForm):
+class FirstBootForm(ValidNewUsernameCheckMixin, EmailFieldMixin,
+                    auth.forms.UserCreationForm):
     """User module first boot step: create a new admin user."""
 
     username = USERNAME_FIELD
 
+    class Meta(UserCreationForm.Meta):
+        """Metadata to control automatic form building."""
+        fields = ('username', 'email', 'password1')
+
     def __init__(self, *args, **kwargs):
         self.request = kwargs.pop('request')
         super().__init__(*args, **kwargs)
@@ -399,6 +404,8 @@ class FirstBootForm(ValidNewUsernameCheckMixin, auth.forms.UserCreationForm):
         """Create and log the user in."""
         user = super().save(commit=commit)
         if commit:
+            self.save_m2m()  # Django 3.x does not call save_m2m()
+
             first_boot.mark_step_done('users_firstboot')
 
             try: