Migrate cancan to pundit [open review]

This is an example of how we could implement our privileges system using Pundit instead of CanCan. I've replaced CanCan only in ProjectsController to make this PR smaller. Doing so we can make a smooth migration analysing and discussing smaller changes and removing cancan only after everything is working properly with Pundit. 

It's important to observe that later refactorings will be necessary to DRY the code after the migration is complete.