Replace fa-tag(s) icons with GitLab SVG icons

What does this MR do?

Issue #225926 (closed)

• Update fa-tag / fa-tags icons to SVG label / labels OR tag depending on use case.
• Update fa-user / fa-users to SVG user / users.
• Update fa-calendar to SVG calendar.
• Update all loaders in issue sidebar to new GitLab UI loader for consistency.

Screenshots

Section	Icons changed	Before	After
Epic show sidebar	calendar, labels, users
Issue show sidebar	user, calendar, labels, users
Issue show sidebar (expanded)	loading icon (not shown in screenshots), spacing around title-content
Tags list	tag
Tag show	tag
User profile	users -> user
Pipelines list	Spacing around tag icon

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

N/A

Security

N/A