[1.8.x] Moved definition of chunks out of timesince function.
[1.8.x] Moved definition of chunks out of timesince function.

This speeds up the timesince function/filter substantially.

Backport of d6969ab from master.

pquentin/django,pquentin/django,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy