Show number of unique fields

[philrz]

At the time this issue is being filed, Zui is at commit 28aede8.

At the bottom of the panes showing the Original/Preview data, there's some summary information such as the number of Types and Rows of the data being staged in the Preview & Load workflow.

In the task list of #2834, @jameskerr noted that an additional useful one to add would be a count of the number of unique fields.