Add indicators to off-site links in wp-admin

[Clorith]

Now that target="_blank" has been removed from links in the help tab on pages, off-site links will take the user away from their current screen.

While a fan of removing _blank links, we should set expectations for users, the tabs on one side open new panels, on the other side they take you to Codex articles on .org while still retaining a similar style.

I can see this as being a frustrating point for users who'd use the help tab and suddenly find them selves not only losing data from their forms etc that may not be fully filled in, but also taken to a completely new domain.

Introducing a new screen reader text alerting users to a link taking you to an external source, as well as an icon or similar for non-sr users would be good in regard to UX.
I believe the link icon of a box with an arrow going diagonally out of it has become a well established indicator of external links, we even have a ​dashicon ready for it

[johnbillion]

Previously: #14099

[desrosj]

Marking for design feedback.

[ibdz]

This was discussed in ​design meeting and it's good to have indicators for off-site links in the help tab on pages, but it should be a different pattern from external link indicators in Gutenberg editor that have target="_blank" (open in new tab).

I think using a ​dashicon in front of external link (as same as in this trac) is a good way to go for indicating off-site links in WP-Admin help section that open in the same tab.