Indicate external link behavior

[aviannamiller]

omeka/Omeka#1108 deals with external link accessibility in core, adding screen reader text and visible icon indicators to links that open in new tabs (or windows). Admin-side, Local Contexts external links pick up on the icon but not the screen reader text. On site pages, neither the icon or screen reader text are present and should be added in.

[kimisgold]

Screen reader text has been added on the a11y/label-external-links branch on this plugin. The external link icon should now appear on the public views as well, but that's a change to the core on the matching branch.