[Project] Introduce publish workflow

This is a high-level feature with multiple CUJs that need to be defined in more detail. The general idea:

* Projects would start out in the test/unpublished state.
* Features that are part of a sampling design would have a special flag to differentiate them from features added by contributors.
* Project managers can "publish" a project, which clears all features and observations added to the project _except_ features flagged as part of the sampling design.
* Once published, questions can't be deleted (or changed?). There may be a manual override for project managers.

Requested by @kdyson. Please cmiiw or update/comment freely.