Main Navigation Bookmarks and Pinning #275

All bookmarks are supposed to be shown in a collapsible (state saved in localStorage) section in the Main Navigation (main sidebar). On each list element, there should be a button to open the URL in a new tab, and clicking the main element outside of the button should open it in an iframe inside the Stackable Cockpit.

Part of this ticket is also to add a pinned section in the Main Navigation and allow pinning of bookmarks from the dashboard as well. Pinned management is done by:

1. Administrators: They can edit a bookmark to make them pinned for every user. On by default.
2. Users: Each user can also pin bookmarks themselves. These are saved in the database on a user basis.