[IMP] base,base_setup: easier multi-company configuration

- The 'Extra Access Rights' is now hidden out of debug mode
- The fields 'company_id' (current company) and 'company_ids'
  (allowed companies) are hidden if there are no more than 1
  company in the database.
- If there are more than 1 company, the fields are displayed
  on the user form view and setting more than 1 company on the
  user will add him into the group 'group_multi_company'. On the
  same way, setting only one company on a user will remove him
  from this group
- The group 'group_multi_currencies' is displayed on the General
  Settings. Enbaling this feature will add all the users into this
  group. This is more accurate as this feature is more global
  than per employee.