Collapsible Navbar

Continuation of #217 

#### General changes

- A smaller version of the logo is used
- Extracted navbar related directives (`navbarNavigation`,  `navbarButtons`, `navbarNotifier` and `navbarSearch`). Right now they only work in the context of the `arethusaNavbar`, as they inherit their visibility status from there.
- The navigational elements in the navbar are prettier and more functional now (using icons, a disabled status instead of hiding them, titles to reveal intent).
- The search section also uses an icon.

#### Collapsible behaviour

`arethusaNavbar` keeps track of the window width. When certain thresholds are undershot, the navbar starts to collapse in the following order:
- Notifier messages are hidden
- The logo is replaced by an icon
- The button group collapses to a single menu button, which triggers a dropdown that contains all elements

After that (~660px width) Foundation starts to hide the complete section part of the navbar (that is, everything besides the title) and goes to something like a full-mobile mode. We probably don't want this anymore, as we handle the collapsing ourselves now, but to communicate with Foundation here, we need to handle its SCSS variables (cf. #184)