Can make highlights on top of external links

[MartinFoys]

Issues with being able to make a text highlight over an external link, and vice versa - needless to say, the ability to do this causes issues once you have two kinds of active areas on top of the same bit of text - one of these active areas cannot respond.

Ideally, you'd not be able to make an external link if the text is highlighted (or the reverse, either option is a fix).