Describe the bug
From what I understood, non-members of a private and transparent assembly should be able to see it but not to participate.
I did a test on staging and as a non-member of a transparent assembly , I was able to create a proposal

Test here : http://staging.decidim.codegram.com/assemblies/test-agora/f/133/proposals/55

https://meta.decidim.org/processes/bug-report/f/210/proposals/12555

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Stacktrace
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Extra data (please complete the following information):

• Device: [e.g. iPhone6, Desktop]
• Device OS: [e.g. iOS8.1, Windows 10]
• Browser: [e.g. Chrome, Firefox, Safari]
• Decidim Version: [e.g. 0.10]: 0.11.x
• Decidim installation: [e.g. MetaDecidim]

Additional context
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