Extend Certificate Master view

[Robin481]

After we have added the option for people to add certificates from the master list #785.

We want to be able to see who has selected which option from the master list.
Similar to the /skills view there should be a number for how many people have that certificate selected that one can click on and see all people.

TODO:

• Add a number next to the certificate master list that shows how many people have selected that certificate
• On clicking on that number open a modal that lists all people that have it selected (similar to the overview in /skills)
• Tests of course 🦪