Add global toggle for unusual hours

[LFBaily]

What?

Allow site editors to easily alert users to unusual hours across the whole venue during periods like Christmas

Why?

For periods like Christmas where users expect disruption to usual opening times so want to be very clear when (and which parts) of the venue are open.

Background

See #10271 and #10640

Done when

• [ ] Add a global toggle in the BE that applies the Unusual open times yellow box to all venues for periods like Christmas and New Year
• [ ] Ensure when the global toggle is off, the Unusual open times yellow box only appears next to the venue that has had exceptional open times added