Replace labels in HAML with gitlab-ui CSS

What does this MR do?

Refactors labels in HAML to use CSS classes from gitlab-ui. The main change of this MR is in labels helpers. It wasn't possible break this down because of the number of places where label helpers are used.

Impacted labels:

• Board sidebar
• Board list title
• Issues list
• Epics list
• Epic page notes (not sidebar)
• Issue page notes
• Issue sidebar (refresh after editing - labels are rendered differently on edit)
• Milestone page - Issues
• Milestone - Labels tab
• Labels list
• Merge requests list
• Merge request sidebar
• Epic & Issue description
• Delete label modal

Other labels are updated in the following MRs: !21465 (merged) !22071 (merged)

Screenshots

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

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 #38144 (closed)