Make UI to show user Work Plans are still loading

[LaChope]

Linked to

• https://github.com/kbss-cvut/aircraft-maintenance-planning-system/issues/142

It is "normal" not to get plans for first few minutes, some interface (e.g. loading wheel) should tell the user that the plans are still loading (and not make him think the app is broken)

@kostobog @blcham