fixing publishable flag to be the same as in StaticContent

follow form layer convention with setter.

fix #70

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes : existing content must be migrated to rename isPublishable property to publishable
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #70
| License       | MIT
| Doc PR        | -