Show user avatars in notifications on Android Pie

...uschku/quasseldroid/service/QuasselNotificationBackend.kt

...u/quasseldroid/service/QuasseldroidNotificationManager.kt

...src/main/java/de/kuschku/quasseldroid/service/SelfInfo.kt

.../java/de/kuschku/quasseldroid/util/NotificationMessage.kt

...java/de/kuschku/quasseldroid/util/avatars/AvatarHelper.kt

app/src/main/res/values-de/strings.xml

app/src/main/res/values/strings.xml

...va/de/kuschku/quasseldroid/persistence/QuasselDatabase.kt