Add (opens in a new tab) hidden text to the cornerstone content link

[afercia]

The new cornerstone checkbox has a label with a link to the relevant post on yoast.com:

<a href="https://yoa.st/metabox-help-cornerstone?utm_content=4.9" target="_blank">cornerstone content</a>

the link opens a new tab/window and we should inform screen reader users of the context change. .I'd propose to add the same, visually hidden text we're using in other places.

I'd also consider a couple more changes, if no objections:

• move the link out from the label: label should be clickable, and this is clickable but also contains a link. This could be potentially confusing, e.g. clicking the link doesn't change the checkbox state as it should
• this would require to modify the label text
• also, links should be meaningful even when read out of context and cornerstone content doesn't help so much to understand where the link points to
• one interesting consideration is that browsers open a new tab or a new window also depending on user settings, for example Firefox has a setting "Open new windows in a new tab instead" which is checked by default... so should we say "tab" or "window"?

Maybe something like: [] <label>Mark this post as cornerstone content</label> <link>Learn more about cornerstone content <hidden text>(opens in a new tab)</hidden text><link> ?

[afercia]

The cornerstone checkbox is now reactified, see https://github.com/Yoast/yoast-components/pull/439 will open a new issue.