Allow Hiding/Collapsing of Milestone header on Roadmap

What does this MR do?

• Add a chevron button next to the "milestone" heading to expand/collapse the milestone list according to the proposed spec (updated)

• Fix the misalignment between chevron and epic label.

• Migrate some of the Bootstrap utility classes used in milestoneListSelection and milestoneTimeline to their @gitlab/ui counterparts.

• Refactored epic_item_details_spec.js by re-organizing into new describes and moving the helpers inside describe.

Screenshots

Demo

Fix the misalignment between chevron and epic label.

(before) (after)

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

Issue #212494 (closed)