Add more FE-required fields to /api/v1/instance

account_activation_required is needed by the frontend to conditionally handle the registration form. It's currently available in /nodeinfo/2.1.json, but this MR adds it to /api/v1/instance too.

When account_activation_required is disabled (default), the user will be logged in immediately after registering. When enabled, the user needs to see a message saying "Check your email to confirm your account"

Related: !2430 (merged) #1710 (closed)

One thing I'm unsure of is whether this should go under "metadata" or not. Technically the entire controller is metadata.