- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

This structure moves closer to the structure template the team decided
on. Components and services are kept in dedicated folders instead of
following the logical code structure. This is still not the case here as
the unit-view component is still nested, but it's closer.