The new links in the News dashboard widget need to inform users they open in a new window

[afercia]

The revamped News dashboard widget (a.k.a "Nearby events" widget) just merged in [40607], introduces three new link that use the attribute target="_blank"

Regardless if links that open in a new window use an icon to give some visual feedback (although the doubt here is about the meaning of the icon: external? opens in a new window? both?) they always need some accessible text to inform screen reader users they open in a new window.

This issue is a long standing one and already discussed in #23432. Recently, we've started removing a few target="_blank" where it made sense, and adding some hidden text, for example in [40422], following the pattern used in [38829].

I'd like to propose to follow the same pattern here and add the same text, with just a small change. The current string is: (link opens in a new window) however the word link is redundant because screen readers already announce link for an a element.

Note that the brackets around the text make screen readers do a little pause, which is a good thing when a link gets announced, for example:
link Meetups {brief pause} opens in a new window.

[afercia]

40733.diff​ does the following:

• standardizes similar messages always using (opens in a new window)
• adds translators comments
• hides the dashicons with aria-hidden="true", see #40428

Safari 10 & VoiceOver need a little trick to work properly. Remove the CSS and test with Safari 10 & VoiceOver, you will notice the hidden text gets read out before the link.

Worth noting this could also use an aria-label on the links, for example:
aria-label="Meetups (opens in a new window)"
but I thought reusing the same string will make life easier for translators.

[afercia]

Refreshed patch using printf().

[afercia]

40733.3.diff​ uses esc_url( __() ) for the "translatable URL".

[afercia]

Screenshot with the patch applied:

[afercia]

In 40643:

Accessibility: Add "(opens in a new window)" screen reader text to the "News-Nearby Events" dashboard widget footer links.

• standardizes similar messages in core to always use (opens in a new window)
• adds translators comments
• hides the dashicons with aria-hidden="true", see #40428

Fixes #40733.