Fix approved project scopes

• Add approved_at to projects, which is set to the initial approval date of the project.
• Redefine relevant scopes in terms of once_approved (approved_at is set) and remove others
• Fix last_approved_version to only return something if there was an approved version
• Add last_approved_version_or_current to perform the old behavior (in case we need to display the last approved version, or otherwise the current version)
• Simplified some permissions
• Add ProjectVersioned to load the versioned project or raise exception if never approved (used in normal interface where the pages for unapproved projects should not display anything).
• Migrate the PaperTrail versions to our new format since we rely on them for the last_approved_version display
• Sort projects overview by approval date rather than creation date

Closes #337 (closed) #345 (closed)