Add an option for a "photo of the day"

[AlstonLin]

The lock screen will check every time it's been locked and it connected to wifi what the day is, and when was the last time the lock screen image updated.
If it need to refresh, then the service will do a request to https://unsplash.it/SCREEN HEIGHT/SCREEN WIDTH/?random to fetch a random picture as the new photo. Note that this photo should save at the same filepath as the old photo it's replacing (and hence it gets deleted) so the disk space doesn't get cluttered.