[Navigation] Add external-link icon when an external link is added to navigation or links portlet

[srenault-meeds]

Is your feature request related to a problem? Please describe.
When adding links to navigation or to links portlet
Then the admin can make it open in a new tab

Users have no clue about the fact it will be opened in a new tab

Describe the solution you'd like
When the link added to navigation is set to be opened in a new tab (internal or external, no matter)
Then a external-link icon is displayed next to the label so the user can see it will be opened in a new tab