Set static gitattributes for design repositories

What does this MR do?

CE backport https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29328

Set static gitattributes for design repositories

The attributes enable all uploaded files to be stored in LFS.

We define static git attributes for the design repository as this repository is entirely GitLab-managed rather than user-facing.

Defining them in the class rather than committing an actual git attributes file allows us to continue to maintain the git attributes without needing to make further changes to a file in a repository if we ever need to add to them.

If ever the design repository becomes user-facing, then we could migrate the git attributes to a file at that point, but there are no plans to do this in future.

See this thread https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13894#note_178002089 and also https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13894#note_178049984

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

Closes #9490 (closed)