Add time since status last changed column to overview

The new column displays the time since an alert's status was last changed to make it easier to see if an alert's status has fluctuated.

Added a corresponding `status_last_changed_at` column to the database to track this value.

In the `perform-check` function, I renamed `last-checked-at-timestamp` to `current-timestamp` to reflect that it is now being used for more than just the `last-checked-at` value.