Use client base url in example pagination

[jvhoffbauer]

Description

When Doccano is hosted behind a reverse proxy, the doccano backend's hostname is something like doccano thus the URLs it generates start with http://doccano:8080. This causes issues when downloading examples using pagination. The next URL will start with the hostname the backend sees.

In case the client cannot retrieve the given URL, it should try using the base_url that it was initialized with.