Add "GitLab Team Member" badge to epic headers

What does this MR do?

See #212259 (closed)

Part of a larger effort to help users identify when they are communicating with a "GitLab Team Member".

Screenshots

Page	Before	After
Epic
Epic Mobile

Local Testing

1. Enable :gitlab_employee_badge feature flag.
   • bin/rails console
   • Feature.enable(:gitlab_employee_badge)
2. In lib/gitlab.rb change self.com? to return true
3. Install an EE license. See https://about.gitlab.com/handbook/developer-onboarding/#working-on-gitlab-ee.
4. Create a user with a @gitlab.com email address
5. Navigate to a Group and create an Epic

Note: You may need to run gdk restart because of the caching on app/models/user.rb

Related MRs

This MR is dependent on

• Issue/MR/Epic comments and notes - !28243 (merged)

Other related MRs

• Issue/MR/Epic list view and Issue/MR headers - !28257 (merged)
• Issue/MR assignee and assignee dropdown - !28377 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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