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.

Also, fixed #20 (unclickable status and pause buttons in Firefox). When the overview table was wide enough, it was overlapping the invisible `aside.side-column` element, and in firefox this `aside` was then intercepting clicks on the status and pause buttons. As a workaround, I lowered the z-index of this element so those buttons would be clickable. In the future, the css could be refactored so this `aside` element isn't necessary.