Consolidate Connection Groups, Types and SubTypes

**Background:**

Currently, there are vastly different objects to "label" connections. Connection Groups, Connection Types, Connection Sub Types. It is very confusing for users and developers what these are, and how are these different.

For example Groups are used in Targets. Types are used in Revenue breakdowns. But this isn't intuitive at all.

**Scope:**

It does make sense to have some kind of labelling mechanism for connections. But one is totally enough.

As part of this ticket, please:

- Consolidate all three existing labelling mechanisms into a single one. This can either be achieved by keeping one of the above (Probably groups, but has to be decided by developer) and merging the other (Types, SubTypes) into this one.
- Alternatively, a new one can be created.
- Remove all leftovers from the to-be-deprecated (Like UI elements, database tables, etc...)
- Find an intuitive name for this object
- Sections in the usage guide need to be adapted to reflect the new logic

**Definition of done:**

There is exactly one labelling mechanism and no leftovers from earlier implementations are present in the code base or UI.