Mentor details - add project

[edmondpr]

Design: https://www.figma.com/file/DgrKZX9mS13Zgf7k5pZ1WT/Partner-web-app?type=design&node-id=727-148&t=LPfyjEb31x9adSCp-4

When clicking on the Add project button, a dropdown menu with the list of available projects (the ones that are not already associated with existing courses) will appear underneath and the logged in user will be able to add for the selected mentor only one project that has no course associated to it.

The /api/v1/partners/{partner_id}/mentors/{mentor_id}/projects endpoint will be used for the list of available projects and for adding the project.