Create Categories manager

[overeha]

Currently the resource categories as well as volunteer specializations are hardcoded.
The app needs a category manager to allow the DSU to add, edit or delete the existing or new categories.
Every category should have subcategories but it is not mandatory.

[sdecianu]

@overeha do volunteer specializations need a manager (similar to the categories manager suggested by this issue), or is it fine for them to be hardcoded for now?

[sdecianu]

By the way, I think I can start on this one - my initial thought is doing it in a similar structure to how the Resources are managed. Not entirely sure about subcategories, though