Added hard coded BASE_URL

Avoid problems with content importer.

To be more specific: Content imported during the installation gets processed by the [translateTo](https://github.com/concrete5/concrete5/blob/master/web/concrete/core/controllers/blocks/content.php#L271) method which turns any http:// links into `{CCM:BASE_URL}` if `BASE_URL` is not set. 

e.g. `<a href="http://www.example.org">` got translated into `<a href="{CCM:BASE_URL}www.example.org">`.