Rework the ExternalLink component

[lorenzo-cavazzi]

ExternalLink should set a standard for rendering links leading outside RenkuLab in a standardized way.
We added too many properties to the current component and customized the look too much -- even having variations like <ExternalLinkButton /> that renders very differently than <ExternalLink role="button" />

We should make a new version of that and use it consistently everywhere in the Renku v2 pages, leaving the styling to the className property as we do for Link (i.e. for rendering as a small primary button, just use the required Boostrap classes).
We should agree with the design team whether we want to include a visual indication of the link leading to an external place (E.G. the Box arrow up-right bootstrap icon after the text) and apply it consistently, with an option to turn it off for exception cases but no ways to customize it (no iconAfter, iconsSize, iconSup, ...)