Add oldest recording highlight in the filter calendar

[sivolc73]

On another DVR software I used it was easy to see when was the oldest record when using the calendar.
The oldest date would be highlighted or the previous date before could not be selected.

It would be a neat feature to estimate how far back I can go without guessing from camera to camera, even just a banner or tag that say "Oldest record: DATE"

[curtishall]

Thanks for the feature request, I like it.