Refactor DataManager and its use

[LoeHendriks]

Currently all data is retrieved at one time, the list of users and then the data per user.
refactor so that only the list of user is gathered at the start and only the data per user once that user is selected from the graph

[LoeHendriks]

@patbro if we want to do this in a nice way I think an update to the backend is needed. Since we want to show the target in the list view as well, can that be added to the data which is retrieved when the list of users is retrieved. Because if we refactor as described above we currently can only show the username in the list