[Feature] List CDPs assigned to a study and studies linked to a CDP

[afymer]

Is your feature request related to a problem? Please describe.

The VPO should be able to quickly visualise the load spread among the administrators.

Describe the solution you'd like

First, a few API routes to list the CDPs of studies:

• /study/[studyId]/cdps to list the CDPs assigned to the study with id studyId
• /administrator/[adminId]/studies to list the studies of an admin

Then, a component to visualize this. See the proposal in #195

Describe alternatives you've considered

Any alternative solutions or features you’ve thought about.

Additional context

The return type of the two list-like API routes should be minimal: ids and only the necessary data for display in the list.