Adds a btn-icon class

What does this MR do?

Not a lot. It just adds a CSS helper class for an icon button in EE. Related discussion.

It's a stop-gap till we refactor the buttons and avoid a lot of the nonsense outlined in #46152 (moved) and our various shades of gray for icon buttons.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.