Allows square avatars, fixes avatars in autocomplete

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

.../java/de/kuschku/quasseldroid/settings/MessageSettings.kt

...rc/main/java/de/kuschku/quasseldroid/settings/Settings.kt

...kuschku/quasseldroid/ui/chat/input/AutoCompleteAdapter.kt

.../kuschku/quasseldroid/ui/chat/input/AutoCompleteHelper.kt

...e/kuschku/quasseldroid/ui/chat/messages/MessageAdapter.kt

...u/quasseldroid/ui/chat/messages/QuasselMessageRenderer.kt

.../de/kuschku/quasseldroid/ui/chat/nicks/NickListAdapter.kt

...de/kuschku/quasseldroid/ui/chat/nicks/NickListFragment.kt

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

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

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

app/src/main/res/xml/preferences.xml

...java/de/kuschku/quasseldroid/viewmodel/EditorViewModel.kt

...e/kuschku/quasseldroid/viewmodel/data/AutoCompleteItem.kt