Replace instances of 'his/her' with 'their'

The his/her construct is, I'm sure we can all agree, nightmarish. It's awkward to read and it's almost impossible to speak aloud. It's also exclusionary and reinforces the idea of a first and second gender.

The contribution guidelines say that if it's anything but a typo, I should file a bug. In this case, it does appear that this is on the level of typo; there are currently no instances of 'he/she' or 'his/hers' in Django, and there are already a decent number of instances of singular 'they', so I'm confident this is not a policy issue. Just in case I'm wrong about that, though, here's a conservative hunt for some examples:

```
$ ack --type-set=po:ext:po --type=nopo "user's" -C 3 | grep their
django/contrib/auth/__init__.py:107:    Removes the authenticated user's ID from the request and flushes their
docs/ref/contrib/csrf.txt-12-who visits the malicious site in their browser.  A related type of attack,
docs/ref/middleware.txt:203:Allows a user's sessions to be invalidated when their password changes. See
docs/releases/1.2-alpha-1.txt-394-malicious site in their browser. A related type of attack, 'login
docs/releases/1.2.txt-147-who visits the malicious site in their browser. A related type of attack, "login
docs/topics/auth/default.txt:114:Changing a user's password will log out all their sessions if the
docs/topics/auth/default.txt-924-    Allows a user to reset their password by generating a one-time use link
docs/topics/i18n/timezones.txt-180-they use the time zone of their primary audience or UTC. pytz_ provides
docs/topics/templates.txt-440-what would happen if the user entered their name as this::
```

Sorry about the .po files; I couldn't find anything in https://docs.djangoproject.com/en/dev/internals/contributing/ about how to run makemessages against django itself.