managed_field:add emitted before DOM has been appended

[revgum]

The callback for managed_field:add is triggered early, and seems like it should happen after the children elements have been appended.

Move: https://github.com/samvera/hydra-editor/blob/master/app/assets/javascripts/hydra-editor/field_manager.es6#L117

after the append:
https://github.com/samvera/hydra-editor/blob/master/app/assets/javascripts/hydra-editor/field_manager.es6#L99