Fix UnicodeEncodeError when sending mails after document edits with unicode characters in the title.

This should fix the following traceback:

    Task tidings.events._fire_task with id f7ef5e0b-6e24-4596-a249-69aa6340b829 raised exception:
    "UnicodeEncodeError('ascii', u'Web\\u6280\\u672f\\u6587\\u6863', 3, 7, 'ordinal not in range(128)')"


    Task was called with args: (<wiki.events.EditDocumentEvent object at 0x41c8ad0>,) kwargs: {'exclude': <User: liminjun>}.

    The contents of the full traceback was:

    Traceback (most recent call last):
      File "/path/to/kuma/vendor/packages/celery/celery/execute/trace.py", line 181, in trace_task
        R = retval = fun(*args, **kwargs)
      File "/path/to/kuma/vendor/src/django-tidings/tidings/events.py", line 132, in _fire_task
        for m in self._mails(self._users_watching(exclude=exclude)):
      File "/path/to/kuma/apps/devmo/email_utils.py", line 154, in emails_with_users_and_watches
        yield _make_mail(locale, u, w)
      File "/path/to/kuma/apps/devmo/email_utils.py", line 70, in wrapper
        return f(settings.WIKI_DEFAULT_LANGUAGE, *args, **kwargs)
      File "/path/to/kuma/apps/devmo/email_utils.py", line 136, in _make_mail
        subject.format(**context_vars),
    UnicodeEncodeError: 'ascii' codec can't encode characters in position 3-6: ordinal not in range(128)

Also renamed some variable to have more than one character in the name.