support user Preferences in general (& specifically for OnFollow)

[cjslep]

This entails:

• Framework allowing app.Application to add and modify arbitrary payloads
• Framework allowing app.Application to modify existing payloads (ex: pub.OnFollow behavior)
• Using OnFollow for the S2S behavior

[cjslep]

I'm thinking of having the arbitrary payload internally just be handled as a json.RawMessage and letting the app.Application supply an arbitrary struct which will be serialized-from/deserialized-into as necessary, so clients can just supply a concrete struct instance. This has the added benefit of delaying handling most of the JSON serialization/deserialization overhead during AP request handling.

[cjslep]

7217bba