Enhancement: Dynamically loaded nav items

We have moved the navItems from application configuration into a store module. We extended it's functionality
by introducing statically and dynamically loaded navItems. This way navItems can be loaded based on extension
data, as soon as the extension becomes active. Please note that having at least one static navItem (coming
from the appInfo object of an extension) is still a requirement for the extension appearing in the app switcher.

https://github.com/owncloud/web/issues/3497
https://github.com/owncloud/web/pull/3570
