Web IDE "Start web Terminal" button doesn't appear when the instance uses a relative url

Summary

When the instance running is using a relative url, the Start web Terminal button in the Web IDE doesn't appear.

In order to show this button, we first perform some checks in the check_config endpoint. But, when the instance uses a relative url, the endpoint address is not correctly built.

The IDE calls http://server/namespace/project/ide_terminals/check_config when it should be http://server/gitlab/namespace/project/ide_terminals/check_config (if /gitlab were the relative url set).

Steps to reproduce

Configure the instance with a relative url, open a project with the Web IDE and check the inspector. A 404 will be raised when calling the check_config endpoint.

What is the current bug behavior?

The check_config endpoint address is wrongly built when the instance uses a relative url.

What is the expected correct behavior?

The check_config endpoint address would include the relative url in its address.

/cc @pslaughter @andr3