'Select All from author' should respect current chat position

[Kylmakalle]

In the mean time the feature selects all the messages from the very bottom of the chat. This may not work correctly if user want to select/delete only a part of the messages for current history.
There're also some other questions regarding this feature - should we select more than 100 messages per once or should we rethink UX of this feature, cause main purpose of it is to delete messages in bulk and it's a bit painful to do that.