Add (and notify) team members that are responsible for training materials#96

As a trainer/coach or team manager/admin, I want to add players that are responsible for the training materials, so that they and the rest of the team know who’s turn it is.

Nice-to-have: notify players responsible x hours/days before each training or match.

Nice-to-have: add option to select the material needed. For instance, number of balls, water bottles, vests, etc