Increase Inverse Navbar Contrast

Address #10809
Lighten navbar-inverse-bg
Increase darken on both navbar-inverse-link-active-bg navbar-inverse-border
Take contrast ratio between .active and navbar-inverse-bg from 1.3 up to 1.5
Keep contrast ratio between in-active links and navbar appropriate for W3C level AA
Screenshots: [before](http://cl.ly/image/3i2H3T1B3y0w)/[after](http://cl.ly/image/3G1F3D121w2c)