Add support for holidays

We send out reminder emails/chats every monday, even if monday is a holiday. The app-wide settings page should let you specify which mondays are holidays, and send the snippet reminders on tuesday instead for that week.

Start by locating the app-wide settings page and the entry point that sends weekly snippet reminders, then trace how Monday delivery is scheduled. Done means users can identify holiday Mondays in settings and that week's reminder is sent on Tuesday instead; verify both email and chat behavior.

Written by the indexing model from the issue text.