Rename `path` to `slug` on Project

[jachro]

We'd like to have a slug property for the current path on the Project entity.

Acceptance criteria:

• add a new slug property to the Project entities across all APIs; the value should be the same as the current path (PR)
• change the modelling so there's no Path type but Slug instead (rename)
  • adapt all the process writing renku:projectPath to TS to also write renku:slug (TRIPLES_GENERATED, ADD_MIN_PROJECT_INFO and SYNC_REPO_METADATA)
  • create DB migrations (EL, TR) to rename project_path to project_slug
  • create a TS migration to copy renku:projectPath to renku:slug in the TS (release)
  • switch all the queries that use the current renku:projectPath to use renku:slug
  • create a TS migration to remove renku:projectPath and encoders that put it into the TS
• create an issue to remove the old path property from all the APIs - once all the teams (UI, CLI, Omnibenchmark) are happy for us to do so.