Add "Promote to epic" issue actions dropdown item

What does this MR do?

This MR adds a new "Promote to epic" button for issues so that users can more easily promote an issue to an epic. Currently, a user can only promote an issue through the /promote quick action.

This change is part of a feature that is behind the feature flag :vue_issue_header

Screenshots (strongly suggested)

Screen_Recording_2020-11-10_at_1.32.43_pm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #233974 (closed)