Unspecced 'sender' field on presence events in /sync

Nobody has claimed this yet.

The [/sync spec](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-sync) claims:

- The `presence` property is of type `Presence`
- The `Presence` type has an `events` property that contains an array of `Event`-type items
- The `Event` type has `content` and `type` fields

However, in practice, the presence events also contain a `sender` field, which is not accounted for in the spec:

```js
// ...
"presence": {
"events": [{
"type": "m.presence",
"sender": "@redacted1:example.com",
"content": {
"presence": "offline",
"last_active_ago": 55595
}
}, {
"type": "m.presence",
"sender": "@redacted2:example.com",
"content": {
"presence": "offline",
"last_active_ago": 38143
}
},
// ...
```

This `sender` property *is* [specified for the `to_device` events](https://matrix.org/docs/spec/client_server/r0.6.1#extensions-to-sync), but that has no relation to `presence`.

Start with the /sync specification's presence events and the linked Event and to_device event definitions. Compare the documented presence event fields with the example, then update the presence event definition so the sender field is accounted for; the issue is done when the /sync spec matches the shown response.

Written by the indexing model from the issue text.