[body upload] Add a file staging preview to the datasetEditor

[chriswhong]

Our current dataset editing UX does not allow the user to see their dataset's body until after they commit. As we are moving to allow the user to edit other components during the creation step, the idea flow is:

• User selects or drags in a file
• The file is immediately uploaded to a staging API, which returns a temporary dataset preview
• This preview is used to display a body table in the body tab, and populate the structure tab
• User can edit other tabs to add metadata, column titles, set types, etc
• User commits their dataset.