percent signs escaping en trans/blocktrans templatetags

[mgu]

When I look at the django source code, I see that % signs are escaped to %% during extraction [1] [2] then replaced by % during the rendering [3].

There is no such replacements during the extraction, but Django will still want to replace %% by %on rendering.

If you confirm my analysis, I can create a PR to fix the issue

[1] https://github.com/django/django/blob/stable/1.11.x/django/utils/translation/template.py#L182
[2] https://github.com/django/django/blob/stable/1.11.x/django/utils/translation/template.py#L145
[3] https://github.com/django/django/blob/master/django/templatetags/i18n.py#L86