Gateway availability & admin + checkout notices to account for EUR currency limitation

[manospsyx]

• [x] Display admin notice when gateway is enabled and shop base currency is other than EUR. Multi-currency stores may want to ignore said notice, so make it permanently dismissible - https://cl.ly/1t0B0v0K421b.
• [x] When currency is other than EUR, make gateway unavailable. Allow this to be changed via filter for multi-currency stores.
• [x] When checking out with a currency other than EUR and Viva is selected (due to filter above), display notice before order creation.