Extract Work Item userPermissions to its own fragment for CE & EE

What does this MR do and why?

This merge request does a few things to handle different schema of WorkItem.userPermissions between EE & CE, and help with separating mock data for CE & EE tests.

• Creates a separate CE & EE fragments for WorkItemPermissions field, where EE version contains blockedWorkItems, required to use for #505826 (closed).
• Updates EE ee/spec/frontend/work_items/mock_data.js such that;
  • It imports several mock objects/helpers from CE mock data spec/frontend/work_items/mock_data.js.
  • Applies EE-specific fields on top of those.
  • Exports those mock objects with same names.
• Updates specs within spec/frontend/work_items to use ee_else_ce_jest alias while importing mock_data.js where we're likely to run into different version of mock data between CE & EE contexts.
• Updates specs within ee/spec/frontend/work_items to use EE-specific mock_data.js.

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Related to #505826 (closed)