markdown: Prevent OverflowError with large time integers.

`<time:1234567890123>` causes a "signed integer is greater than
maximum" exception from dateutil.parser; datetime also cannot handle
it ("year 41091 is out of range") but that is a ValueError which is
already caught.

Catch the OverflowError thrown by dateutil.

zerver/lib/markdown/__init__.py

@@ -1395,7 +1395,7 @@ class Timestamp(markdown.inlinepatterns.Pattern):
         time_input_string = match.group("time")
         try:
             timestamp = dateutil.parser.parse(time_input_string, tzinfos=common_timezones)
-        except ValueError:
+        except (ValueError, OverflowError):
             try:
                 timestamp = datetime.fromtimestamp(float(time_input_string), tz=timezone.utc)
             except ValueError: