add 'enabled' flag to buildpacks

Disabled buildpacks will not be included in staging messages sent by the
CC. This will be implemented in a later commit.

[#57673254]

Signed-off-by: Johannes Tuchscherer <jtuchscherer@pivotallabs.com>
exclude disabled buildpacks from staging message

moved the logic for staging message generation into the app stager task
and out of the model.

[finishes #57673254]

Signed-off-by: Johannes Tuchscherer <jtuchscherer@pivotallabs.com>
stop using standard_model_object

Signed-off-by: Johannes Tuchscherer <jtuchscherer@pivotallabs.com>