[ENHANCEMENT] Check for more days of images than "Days to Keep"

[EricClaeys]

During installation, if "Days to Keep" is smaller than the number of saved days images in ~/allsky/images, the user should be notified that the "extra" images will be deleted in the morning, and then be prompted to changes Days to Keep.
If they don't, a reminder message should be added to the WebUI.

Whenever a user decreases "Days to Keep", the same check should be made and a message added to the WebUI if needed.