Fix epic dropdown not opening in swimlanes sidebar

What does this MR do?

For board_sidebar_epic_select.vue

• Resolves #320876 (closed)
• If user selects an epic that's already assigned to the issue, do not commit the change to the server.

for board_sidebar_milestone_select.vue

• Collapses BoardEditableItem when GlDropdown emits hide event rather than listening for BV_DROPDOWN_HIDE.

  For context, refer to !53686 (comment 504639333).

Screenshots (strongly suggested)

No visual or functional change

Screen_Recording_2021-02-09_at_3.04.42_PM

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (this is an unreleased bug!)
• 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