Turn administration tabs into separate routable pages

Right now the administration page contains multiple sections related to different entities. This makes it difficult to implement more refined routing.

We should replace the numerous tabs with separate routable pages.

The routing of separate sections could either happen:

• At the root of the app - e.g. /nodes, /users;
• One level deeper, within the /administration section - e.g. /administration/nodes, /administration/users.

Similarly, when it comes to the navigation menu item, we could either:

• Replace the Administration item with single entries;

  

• Group the single entries inside a collapsible Administration item.