Add variant support for Epics dropdown

What does this MR do?

This MR adds support for variants in Epics dropdown, it currently supports 2 possible variants;

• sidebar (Default): This renders the component the way it looks in Issuable sidebar (with title and Edit button to reveal dropdown).
• standalone: This renders the component as a dropdown button clicking on which shows the dropdown container with search field

Screenshots

sidebar	standalone
Screen_Recording_2020-05-19_at_6.08.37_PM	Screen_Recording_2020-05-19_at_6.04.42_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

#13847 (closed)