Add coming soon to events, hide past events

What

• Add gray button - "Coming soon!" - (unclickable) when event_url is not present
• Hide events automatically when date has passed (add date_ends variable to events.yml)

@EmilyKyle how about that?

@rspeicher would you mind reviewing for us? 😬

Review Apps link: https://add-coming-soon-to-events.about.gitlab.com/events/