Support for epics in events API

What does this MR do and why?

• Adds the ability to query Epics in the /events and /users/:id/events REST API endpoints.
• Prior to this, the EventsFinder had no EE-specific overrides to Epics were never included in the output.
• Refactors Event::TARGET_TYPES to a class method so that it can be overridden in EE. 560f414d

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Relates to #337310