Overview pages loading

Is your feature request related to a problem? Please describe. When loading an entity's overview page, there is no loading displayed. This is no problem if the connection with the perun backend is good and perun is responding fast. That is not always the case. Because of that, when browsing through the app, it might look to users that something went wrong because they can see only a white screen.

Describe the solution you'd like It would be nice to display a spinner when the app is loading entities' data on overview pages.