[client] Translate the character "%" correctly

xgettext escapes the character "%" in {% trans %} template tag but
Django doesn't recognize it.

https://code.djangoproject.com/ticket/11240

To avoid it I use another syntax for it:

{% trans "% Done" %} ->
{{ _('% Done') }}
Merge pull request #748 from project-hatohol/fix-translate-percentage-2

[client] Translate the character "%" correctly