Allow admin to change "allow public registrations" setting after install

[lightnin]

During the install phase Pleroma asks the admin installing:
"Is the registration open for new users?"
If yes, new users without Yunohost accounts can register. I just found 4 spam accounts on my system who made use of this feature. I would like to disable registration by non-yunohost users, and only permit new pleroma users to be created when Yunohost users on the system go to Pleroma - as in, 1 pleroma account for each Yunohost user.

But in the admin settings dialog in Admin interface on Yunohost, there is no way to change this setting. So I don't have a practical way to stop the spammers from registering on my instance.

(Open to workarounds...)

[lightnin]

It looks like the value that should be configurable is:
":instance: registrations_open: Enable registrations for anyone, invitations can be enabled when false."

https://docs-develop.pleroma.social/backend/configuration/cheatsheet/