WIP: POC for segregation of GL Com code

What does this MR do?

With increasing number of code specific to .com only, there has been instances where .com code has spilled into self-managed:

• https://gitlab.com/gitlab-org/fulfillment-meta/issues/34
• gitlab-com/gl-infra/delivery#483 (closed))

This MR aims to address this by segregating .com into its own module which would be mounted only if it's a .com environment.

It follows the hierarchy similar to what's already there in for ee:

This POC experiments with the existing username_suggestion api by moving it from ee code to com module.

Note: The failing specs are not addressed as yet - as this MR is under ideation review.

Closes customers-gitlab-com#589 (closed)

Screenshots

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