Leverage new openInNewTab prop of KExternalLink

[indirectlylit]

Observed behavior

We're adding a new prop which allows external links to set whether or not they open in a new tab: learningequality/kolibri-design-system#137

KExternalLink is slightly misnamed: it really means "not a Vue router link". It is used for both "internal links" e.g. "Explore without account" on the sign-in page as well truly external links such as to learningequality.org from the privacy policy.

Expected behavior

We should audit our use of KExternalLink and set openInNewTab=true when appropriate.

User-facing consequences

Internal links should stay on the same tab, and truly external links should open in a new tab.

Context

0.14.4

[YashJipkate]

Hi! I would like to work on this issue and have opened a working PR towards this. Please take a look.

Thanks.