CE backport for gitlab-ee!13389 (Support storing design blobs using LFS)

What does this MR do?

This backports to ce a refactor of the shared example 'a controller that can serve LFS files'.

The ee MR that contains the original refactor is https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13389

The reason for the refactor the removal of the outer context as it prevented let(:lfs_oid) from being overwritten and also was not needed.

The shared example was also renamed to be more descriptive.

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