start STOTMEU banner now

note that start date was mandatory in initial per https://github.com/openstreetmap/openstreetmap-website/blob/master/FAQ.md#how-do-i-create-a-banner-to-promote-my-openstreetmap-event

> The pull request must indicate when the banner should start being shown, which must be at least 7 days after the pull request was created.