openExternalLinkInNewPage prop in utils/router.jsx

Now it only affects external links, which are rendered as `<a>` elements.
It makes more sense to ensure that if this prop is given, we enforce `<a>` element even for links within the app (so that the target page will be open in new tab, where the app will be started from zero).
Along with this change, it also makes sense to rename the prop accordingly.