[dashboard] Use user site permissions for dashboard charts

[CamilleBeau]

Brief summary of changes

This PR uses the user sites instead of a full site list in the visualizations of statistics displayed on the dashboard.

• [x] Have you updated related documentation?

Testing instructions (if applicable)

1. Test the dashboard charts with a user who only has access to some but not all sites
2. Make sure that the numbers are correct
3. Repeat steps 1 and 2 but with a user who has all sites
4. Repeat steps 1 and 2 but with a user who has the "all sites" permission