Reject inactive user password recovery

If you try to reset the password from an inactive user, you'll be able to do it, but you'll not be able to login anyway.
Therefore I created a check inside the form "clean_user" field. If the user is inactive and the app settings `RECOVER_ONLY_ACTIVE_USERS=True`, then an error message will be displayed saying that an inactive user can't recover his password.

By default `RECOVER_ONLY_ACTIVE_USERS` is set to `False` in order to make this change backwards compatible.

All translations need a new string translation.
I'd rephrase this to `Sorry, inactive users can't recover their password.` to make the sentence more neutral :)