model's Resource.readonly only works for types, not instances

[duglin]

Setting Resource.readonly to 'true' is fine if all instances of that Resource are read-only, but what if only certain instances are? Or what if just certain Group instances have read-only resources?

Proposal:

• Remove the "readonly" aspect from the model
• Define a "readonly" attribute (bool) for Resources
  • Defaults to "false"
  • system managed, "true" = user can't touch