Allow to limit who can create public and open conversations

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Is your feature request related to a problem? Please describe.
In some situations (schools) it might be okay for users to be able to create group conversations but not public/open conversations.

Currently there is only a single setting:

• Limit creating a public and group conversation

We would need to split this into 2:

• Limit creating group conversations (basically the current one)
• Limit creating public and open conversations (new, but should copy config value from above on migration)

Start by locating the existing setting that limits creation of public and group conversations and trace where its configuration is read and migrated. Add the separate restriction for public and open conversations while preserving the existing group-conversation behavior. Done means the settings are independently enforceable and the new value is copied from the old one during migration.