Feature toggle: hide from admin Iniitiatives, Conferences, Assemblies, Participatory Processes

As a sysadmin administrator, I can:

1. Install a decidim with initiatives, assembly, participatory-process
2. On a tenant "lausanne", disable initiative
3. On a tenant "neuchatel", disable conference
4. Then Lausanne won't see initiative admin button
5. And Neuchatel wont see conference

Tasks

• Add checkboxes in /system to disable participatory spaces if they exists (Look at Decidim.participatory_spaces_registry).
• Add a engine initializer to remove buttons from admin