[NavMenu] Don't open in new tab/window

All other barclamps open in the same tab/window, so we should be
consistent here. General rule is that if it's in the same domain, don't
open a new tab/window.

As requested in
https://github.com/crowbar/barclamp-swift/pull/99#issuecomment-20192359
Merge pull request #32 from jamestyj/release/pebbles/master-same-tab

[NavMenu] Don't open in new tab/window