This change adds the following new resource implementations:

- DefaultReviewerItemResource

- DefaultReviewerListResource

- ExtensionItemResource

- ExtensionListResource

- ReviewGroupItemResource

- ReviewGroupListResource

- ReviewGroupUserItemResource

- ReviewGroupUserListResource

- UserItemResource

- UserListResource

- UserFileAttachmentItemResource

- UserFileAttachmentListResource (split out from old

  FileAttachmentListResource implementation)

- ValidationResource

Ran unit tests.
Used the new resources from a test script and saw that everything

  worked as expected.