make unread conversations float at the top of conversation lists (including widget) via a new setting "Unread at the top"
satisfies #41 and #208
and largely satisfies #99 (but continues to also show read messages below unread messages in the widget)

order of conversation list items with the setting turned on is;

• unread
• pinned
• draft
• last message date

per conversations repo code;

    val sortOrder: MutableList<String> = arrayListOf("pinned", "draft", "lastMessage.date")
    val sortDirections: MutableList<Sort> = arrayListOf(Sort.DESCENDING, Sort.DESCENDING, Sort.DESCENDING)
    if (unreadAtTop) {
        sortOrder.add(0, "lastMessage.read")
        sortDirections.add(0, Sort.ASCENDING)
    }