Fix People section a bit
This does two things:
- Sets `editable` to true for the "People" `RoomSubList` so that people can be dragged to favourites. This has the downside that you can drag a "People" to "Rooms", but it won't set the direct chat flag. This is because im.vector.fake.direct != m.direct, sadly.
- Sets `alwaysShowHeader` to `true` so that the `IncomingCallDialog` can be showneven when there are no people in `sortedList`. Fixes https://github.com/vector-im/riot-web/issues/2956.
