Allow actor id to be specified in config

This is useful if the actor won't be at the root of the domain.

data/tests.toml

@@ -5,6 +5,7 @@ admin_password = "$2b$12$OwCyZM33uXQUVrChgER.h.qgFJ4fBp6tdFwArR3Lm1LV8NgMvIxVa"
 name = "test"
 summary = "<p>Hello</p>"
 https = false
+id = "http://localhost:8000"
 icon_url = "https://localhost:8000/static/nopic.png"
 secret = "1dd4079e0474d1a519052b8fe3cb5fa6"
 debug = true