UI: Use health tests to check if connection to s3gw is valid

[votdev]

This is a follow up feature request of #823.

The idea is that health tests are used to check if the connection to the s3gw is working. Alternatively it would be great to check the setting in S3GW_SERVICE_URL during container startup. This has the limitation that this test is only done once at startup; but can be used to validate the specified URL is valid.