[F] Add soft deletion to projects and texts

[scryptmouse]

• This adds logic to background the deletion of models that can take a while to actually delete because of the complexity of their association hierarchies.
• When soft-deleting a project, its texts will also be soft-deleted while the project is being deleted in the background, so that any lookups on the text will 404 as we'd like.
• Update controllers, filterable concern to use soft-deletion scopes when applicable
• Add TimestampScopes helper
• Add some missing inverse associations, minor idiomatic cleanup

Resolves MNFLD-937