Indicate when a navigation link will open in a new tab/window

Currently there are links in our '?' dropdown that:

• pop open a new tab/window (See what's new at GitLab and Contribute to GitLab)
• go to another page within the GitLab instance (help)
• use the same window to open an external link (Support and Submit feedback)
• open a modal (Keyboard shortcuts).

Since almost all nav items in GitLab open in the same page, it might be a nice UX to indicate when something opens in a new tab. That way the user knows they can click that item and not lose their current place.

I'm not sure if they should apply to items that open in a modal. It's worth considering since the result is the same - the user doesn't need to be concerned about losing their place.