Fixes archived chats screen, significantly improves its performance

...hku/quasseldroid/ui/chat/add/query/QueryCreateFragment.kt

...e/kuschku/quasseldroid/ui/chat/archive/ArchiveFragment.kt

...uschku/quasseldroid/ui/chat/archive/ArchiveListAdapter.kt

...e/kuschku/quasseldroid/ui/chat/archive/ArchiveListItem.kt

...kuschku/quasseldroid/ui/chat/buffers/BufferListAdapter.kt

app/src/main/res/layout/chat_archive.xml

app/src/main/res/layout/widget_archive_placeholder.xml

app/src/main/res/layout/widget_header.xml

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

...ava/de/kuschku/libquassel/util/helper/ObservableHelper.kt

...va/de/kuschku/quasseldroid/viewmodel/data/BufferStatus.kt

...u/quasseldroid/viewmodel/helper/ArchiveViewModelHelper.kt

...chku/quasseldroid/viewmodel/helper/ChatViewModelHelper.kt

...u/quasseldroid/viewmodel/helper/QuasselViewModelHelper.kt