Use the appropriate std::chrono type for the thresholds

Use the std::chrono::duration "days" as type for the thresholds. Since
std::chrono::days exists since C++20 we add our own typedef.
Internally, also always use days instead of a mix of seconds and days by
using an Expiration type.
Additionally, change the meaning of the UI texts "... expired n days ago"
to mean "n full days ago".