ServerFormatter changed incompatibly in Django 2.1

[Jacob Walls]

While developing djangoproject.com, a failing request spewed a large stacktrace to my terminal, as described ​here. Root cause was: that project's logging config extends Django's, and it never caught up with a backward incompatible change in Django 2.1 (#28810) that meant the injection of the server_time variable would only happen for f-string format strings, no longer for %-style format strings.

This breaking change was mentioned during triage but didn't get a response during review.

Of course, I could fix this in djangoproject.com, but it's just as easy to fix it in Django where other projects could benefit.

Proposal: this util:

    def uses_server_time(self):
        return self._fmt.find("{server_time}") >= 0

Should not assume f-string style formatting.