[#2046] Defaulted pleroma/restrict_unauthenticated basing on instance privacy

Defaulted pleroma/restrict_unauthenticated basing on instance privacy setting (i.e. restrict on private instances only by default).

Note: private instances' admins will need to explicitly define config :pleroma, :restrict_unauthenticated in prod.secret.exs if they want to provide API access to specific resources for unauthenticated users. Added the info to changelog, updated the docs.

Closes #2046 (closed)