Make sure ‘&’ won’t be used for new ref types in CE

What does this MR do?

It adds epics to the reference_extractor and defines classes that are necessary so that it doesn't fail. The reference extraction won't work in the CE, epic will be available in EE only.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We are introducing Epics for EE and & will be used as a reference prefix. We need to make sure & won't be picked for new CE references in future

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

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