fix(links): Add "opens in new tab" accessible text

[KevinEtchells]

For links that open in a new tab or window, it is advisable to give users advance warning of this. This is from an accessibility perspective, where if this happens unexpectedly this could be disorientating for some users. Please see further information around this here:
https://www.w3.org/TR/WCAG20-TECHS/G201.html

A small step to improve this would be to add some accessible text to warn users of this. This would be announced by screen-readers, but not visibly shown. E.g.

<a href="http://example.com" rel="noopener noreferrer" target="_blank">my link</a>

Becomes:

<a href="http://example.com" rel="noopener noreferrer" target="_blank" aria-label="my link (opens in new tab)">my link</a>

This only applies if the openLinksInNewWindow option is set.