Transform projects URI field as a machine name

Problem/Motivation

There is actually an URI field required for projects entities. We can put whatever we want there but there is no duplicate prevention.

Steps to reproduce

Create a project with the flag URI field.
Create a second project with the same URI field.
You get a SQL fatal error for value duplication.

Proposed resolution

Transform this field as a machine name with a proper constraint to avoid duplication beforehand.

Remaining tasks

- Add constraint.
- Maybe transform as a proper machine name type field to benefit from the the Title field completion populating the URI field.

User interface changes

Extra documentation / a clearer label about what this field does could be helpful even if most of those data will be filled by automatic imports?