i18n: External link icon should be mirrored in RTL

[rachelmcr]

The external link indicator should be mirrored in RTL.

The icon seems to refer to opening a new window or a new tab, or perhaps to moving forward and out at the same time.

In an RTL environment, the new window or tab would open to the left of the current window or tab, and moving forward would mean moving left. So the arrow in the icon should point left.

Pointing right in an RTL environment suggests going back, cancelling, reverting, etc.

This should be a quick fix in CSS, with .is-rtl to add a different icon, or flip the icon.