Make customer portal URL configurable via environment variable

What does this MR do?

~"group::fulfillment" requires a configurable connection between GitLab.com and the customer portal. Prior this merge request we had to change that code by hands and the problem is that we had to stash (or create a patch) changes later for updating our local GitLab.com copy. You even can forget about changing this URL sometimes.

This merge request adds the ability to set up the URL of the customer portal via an environment variable. This will make the development process easier.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team