Add 'external' icon for external links

[tiiavalt]

Is your feature request related to a problem? Please describe. Currently there's no way of knowing whether link is internal or external until user clicks it.

Describe the solution you'd like Let's add an external icon to each external link to indicate that it will take user outside of Epi.

-- The result

• Created a new component (ExternalLink) that has the external icon right after link's label. It also has aria-label "external link), so external links can be identified by the screen readers as well.
• Extra improvement: added default font for css font-family rule to fix warnings.