FF: Pending state in Import Flag modal while validating json

[danoswaltCL]

On successful drag end or on successful user-select (i.e. a json file was able to be parse), a call will go out to /featureflags/validation. While that call is pending:

• the large draggable "Import" widget will be hidden
• a spinner will show in the middle of the modal (retain the same height as when showing the widget though so it's not janky and popping up and down in height)
• when the response has been received, the spinner will disappear and the next state will be dispatched.

[zackcl]

The load-spinner sounds good to me.

[bcb37]

QA: this looks good.