feat(alert): implement alert component

This implements the alert component, described by #454 (closed).

Old description

This implements the alert component, described by #454 (closed), with the exception of the correct variant-specific icons.

Currently, intentionally-ugly placeholder icons are rendered instead, since they can only be correctly implemented once #98 (closed) is resolved. This is tracked in a follow up issue: #491 (closed).

Screenshots

Old screenshots

Older screenshots

TODO

• Remove getSvgIconPathContent() helper, since it's not supposed to be used in components (blocked by #98 (closed))
• Preserve appearance with and without GitLab CSS (font-family notwithstanding)
• Add another story or two (for visual regression testing)