Add Event#location and refactor the map_marker partial.

It may be longer-winded, but I'd vote for the clarity of this, which avoids __send__ and a rescue:

```ruby
self.lat = data[:lat] if data[:lat].present?
self.lng = data[:lng] if data[:lng].present?
self.radius = data[:radius] if data[:radius].present?
self.speed = data[:speed] if data[:speed].present?
self.course = data[:course] if data[:course].present?
```