Add support for denominators to throttle strings

[awiebe]

Description

Improved denominator string parser so you can for example write 1/30s for 1 request every 30 seconds instead of being limited to throttling by to the closest time unit. Since the current code just uses a lookup table on the denominator, it's pretty straightforward just to jiggle the indexing to look at the end of the string for the suffix, and see if there is a number in the denominator.

I imagine some documentation would also need to be updated. But I figured I would get the ball rolling since it seems a straightforward feature.

[awiebe]

Ok so it turned out to be a little more complicated than just fiddling with string indexes, since units can be spelled out. But a simple regex does the trick.

[sevdog]

This resembles #9079

[tomchristie]

Appreciate the input, tho... https://github.com/encode/django-rest-framework/blob/master/CONTRIBUTING.md

At this point in its lifespan we consider Django REST framework to be essentially feature-complete. We may accept pull requests that track the continued development of Django versions, but would prefer not to accept new features or code formatting changes.