WIP: Edit project customer dropdown

In #147 (closed) it was proposed and agreed that editing/creating customers would be moved to a separate form, and that edit/create project would use a dropdown for selecting from existing customers

@gaelleM I've added the dropdown in this branch and assigned it to you for CSS, if that's okay :)

For it to work you need to update your config.json, by copying it from config.sample.json (I added a customers endpoint)