[RT] [FE] Issue epic link widget supports websockets

Implement the front-end portions to receive real-time changes to the epic link for issues in the issue sidebar.

Steps to complete:

• Add a new GQL subscription: ee/app/assets/javascripts/sidebar/queries/project_issue_epic.subscription.graphql. This requires the respective subscription to exist in the backend. It should query for only the relevant fields.
• Add this subscription query to issuableAttributesQueries.
• Extend sidebar_dropdown_widget apollo config with subscribeToMore impl (this should emit an update event so the widget re-renders). This must also provide a skip implementation so that we only do this on the Issue page and for the Epic link (see this comment).
• Guard these changes with a FF (see gl_feature_flags_mixin).