"Custom" option in the date dropdown

[kiriDevs]

Some (very few) times, substitution plans are published way ahead of time. While these might not necessarily be relevant at all points, I could see people (myself) being curious and wanting to check before it actually is to see what the school is up to again.

We should probably use window.prompt to stay consistent with the window.alerts that the StorageManager emits, as well as the window.confirm the "Reset all settings" button emits.

Also, then we will have all three JS PopUps used.

[kiriDevs]

Maybe we could also just make this optional in settings so the whole thing stays extremely clean by default, but can also have more features enabled when wanted.