Navbar dropdown is hidden behind map

[simonihmig]

Proposed solution: z-index: 1000000 (the more the better 😬)

Seriously though, idk why the leaflet has to use such a high z-index, but that seems to come from the addon's vendor css...

[mansona]

This change is probably best added to https://github.com/ember-learn/ember-styleguide if anyone wants to take this up 👍