I can comment as a normal user in a private assembly

**Describe the bug**
As a normal user (no private user) I can comment on a private assembly where is available. 

**Expected behavior**
I expect that if i'm not a private user I can't participate.

**Extra data (please complete the following information):**
 - Decidim Version: 0.16

**Additional context**
https://meta.decidim.org/processes/bug-report/f/210/proposals/14247
