Refactor EpicToken to use BaseToken

What does this MR do and why?

Refactor EpicToken to use BaseToken for new filtered search

Migrates fetch of epics for dropdown to GraphQL on issues list (behind :vue_issues_list feature flag).

Prep work for #337685 (closed).

Follow up MR: !70476 (merged)

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Related to #337685 (closed)