Clarify language in installation prompts about pleroma_ynh HOT 1 OPEN

Pleroma installer asks:
"en": "Is it a public application?",
Since it is necessary for pleroma to be a public application for federation to work, which is a core feature of the app, I suggest we remove this feature and automatically make all new installations "A public application" without asking the user to indicate this.

I disagree, someone could install a pleroma instance without federation ...

Installer also asks:
"en": "Is the registration open for new users?"
This should be clarified to resolve questions described in 133, for example:
"Should registration be open to users who do not have a Yunohost account on the system?"
It might be worth clarifying that allowing registration from non-yunohost users will make it possible for spammers to create accounts on the system.

Done at 747c2b9

from pleroma_ynh.