Re-implement <Navbar> component following new design guidelines

[emmenko]

Just so that we can keep track of it.

Background

The current implementation of the <Navbar> is pretty "old" and has not really been updated since years (layout and styles).

The design team did provide some improvements in the design and layout, also in regards to having more menu items (for Custom Applications). However, we never really prioritized the work here.

Task

The <Navbar> component should be re-implemented following the new design guidelines.
Furthermore, we should use emotion and not css-modules. This is important as it currently blocks the work in #1871 (related to loading css files via babel).

Design guidelines

https://commercetools.invisionapp.com/overview/MCD-1429-MC-Navigation-v-2-0-ckem9zr8kyl2m01zt0vzg7kqs/screens