Toggling `Timestamp` causes "Not paid" to disappear in `Paid On` column

[dunkOnIT]

• In the Registration Admin page, with payments enabled, see an unpaid user's entry - it says "Not paid" under the "Paid On" header [^Fig 1]
• However, when toggling the "Timestamp" selector, it changes to displaying a timestamp under "Paid on"[^Fig 2] - presumably this is the registration timestamp?

Fixes needed:

• With "Timestamp" toggled on
  • [ ] It should be clear whether we're displaying a payment timestamp or registration timestamp
  • [ ] It should still be clear that a user has not paid (ideally, we still display "Not paid" text) - perhaps this should be displayed under the "Amount" header?

[^Fig 1]

[^Fig 2]