Fix 500 when providing the uuid for a sample in the previous_revision_uuid field for create_entity

[DerekFurstPitt]

In entities/<entity_type> (create_entity), for the field previous_version_uuid, the code allows for this uuid to be for either datasets or samples.

# Make sure the previous version entity is either a Dataset or Sample
if previous_version_dict['entity_type'] not in ['Dataset', 'Sample']:
    bad_request_error(f"The previous_revision_uuid specified for this dataset must either be a Dataset or Sample"

However, if the uuid for a sample is given, a 500 will be raised when the following section of the code is reached:

# Only published datasets can have revisions made of them. Verify that the status of the Dataset specified
# by previous_revision_uuid is published. Else, bad request error. 
if previous_version_dict['status'].lower() != DATASET_STATUS_PUBLISHED:
    bad_request_error(f"The previous_revision_uuid specified for this dataset must be 'Published' in order to create a new revision from it")

Since samples do not contain status field, the error occurs. Either move this status check under a conditional if previous_version_dict['entity_type'] == "Dataset" or modify the earlier conditional to only include datasets and not datasets or samples. Previous revision should never be a sample so this seems to be the better option.

Unless something entirely different was intended, in which case this whole section may need modified.