Add schema:description to Project metadata

[Panaetius]

We should add a schema:description field to the Project metadata to allow moving away from using Gitlabs description field.

This means

• allowing description to be set when creating a project (through renku init and the service)
• adding a renku project edit command that allows changing the description (and maybe other fields as well?)
• Adding a project.edit endpoint to do the same in the service
• Moving some of the other project related endpoints from cache.* over to project.* in the service. We should keep the old ones around for backwards compatibility and mark them as deprecated.

[Panaetius]

refactoring will be done in #2222