Display multiple upcoming events

Adds the ability to display multiple upcoming events on the homepage. In our
case, and as implemented by these changes, we will display 2 upcoming events.
The static `Event.upcoming` method now accepts a parameter, `count`, to support
returning multiple upcoming events. `count` defaults to 1. Adds or updates
appropriate tests, and slightly changes view logic in the
`home/index.html.erb` view.