[Rails] When super-admin deactivates public community feature flag for community, turn off for them

[rudokemper]

Explore Communities added the ability of a new feature flag: public_communities.

When this is enabled for a community, a logged in admin for that community will see a new set of options in their Settings menu:

This enables them to opt in to index their community on public (for example, https://explore.terrastories.app/).

When a super-admin turns off the public_communities feature flag for them, then the menu options in Settings will disappear. However, they will still be indexed on public (and they will still have a slug).

Let's make it so that if a super-admin turns off public_communities for a community, then index_communities_on_public is disabled. (I think the slug can be left alone.)

Acceptance criteria:

• Logged in super-admin turns on public_communities for a community
• The community admin opts in by checking "Enable public interface"
• Super-admin deactivates public_communities for that community
• Now, the community is no longer public e.g. current_community.public is set to false.