Include thumbnails in push notification previews

Turns out that iOS allows any old app to do custom notification UI these days (like iMessage and Twitter do to how image thumbnails). This is described in https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/ModifyingNotifications.html#//apple_ref/doc/uid/TP40008194-CH16-SW1. It would be wonderful to have this as a feature.

Start by reading Apple's linked documentation on modifying notification content, then locate Element's current push-notification handling. The feature is complete when notification previews can display image thumbnails through the iOS custom notification UI.

Written by the indexing model from the issue text.