[#3023] Add new extension points to pass the package dict

When creating or updating a dataset, extensions can hook to the
`after_create` and `after_update` methods to receive the validated data
dict. In the create case, the id is added to the dict.
Merge branch 'master' into 3023-data_dict-extension-points
[#3023] Pass context object to extensions on after_create and after_update
[#3023] Add new after_show extension point

Gets the package dict just before being sent back to the controller.
Removed an out of date comment on tests.
 [#3023] Add new after_delete extension point

Gets the context adn data_dict (tipically with just the id) after
deleting a dataset.
[#3023] Fix failing test
Merge remote-tracking branch 'origin/3023-data_dict-extension-points'