Add params to place text above or below the login form

This PR simply adds 2 plugin parameters ``loginFormTopCell`` and ``loginFormBottomCell`` that can be used to add some custom text above or below the form panel inputs. Could be used to replace custom plugin "CartorivieraLogin".

Example of config:
```
{
    ptype: "cgxp_login",
    ...
    loginFormTopCell: {
        html: 'foo',
        xtype: 'panel',
        unstyled: true,
        height: 50
    }
}
```