Add element and event labels to DOM events

Parses all events where `$event_name` is `"$dom_event"` and adds `$element_label` and `$event_label` as extra properties. Example:

```
{
    "$dom_event": [
       [
           timestamp,
           group_key,
           ip_address,
           {
               "$event_name": "$dom_event",
               "$element_label": "form#signup.horizontal-form",
               "$event_label": "DOM: submit on form#signup.horizontal-form",
               "type": "submit",
               "tag_name": "div",
               ...
           }
       ],
       ...
    "events": [
       [
           timestamp,
           group_key,
           ip_address,
           event
       ],
       ...
    ]
}
```