Replace first_message and last_message with a more up to date version

[tarsil]

Is your feature request related to a problem? Please describe.
I came to realize that the first_message functionality is limited to an old version of Django. Since django 1.11 we can use a different and more cleaner approach such as earliest and latest.

Describe the solution you'd like
Replace the first message with

    def earliest_message(self):
        """
        Returns the earliest (first) message of the thread
        """
        try:
            return self.messages.earliest('sent_at')
        except Message.DoesNotExist:
            return

And the same for the last message

    def latest_message(self):
        """Returns the latest message of the thread"""
        try:
            return self.messages.latest('sent_at')
        except Message.DoesNotExist:
            return

This is useful because we could even pass the default sent_at and allowing the developer to choose a different field at choice.

Using the first() and last() methods from the queryset but performance-wise, is slower.