Add option to prevent access to options early

[slickscreen]

Unless you lock up the whole extension, there currently isn't a way to allow a set of sites without allowing that set to be edited.
This PR adds a way to lock option sets a certain amount of time before the set goes into effect.

It seems to work in its current state, but I'll be doing more testing later. let me know if you want anything changed.

Potential issues:

• Its easy to permanently lock an option set. I don't know the best way to handle this, as the issue is not unique to this.
• It might act strangely on blocked days bordered by unblocked days. Probably fixable, if I change how certain things work.

[slickscreen]

As far as I can tell, this all works now. As with before, let me know if anything should be changed.

[MouseAndKeyboard]

I'm just a random passer-by, but: because you mentioned "Its easy to permanently lock an option set", maybe having some sort of password to access the settings early (you could give that password to a commitment buddy or whatever).
Anyway, I think your feature would be useful :)