Add support for page_size parameter in CursorPaginator class

[cypreess]

CursorPaginator receives identical support for page size query parameter as PageNumberPagination.
Solving #5249

[tomchristie]

Hiya! The code looks good, but I'm still in two minds about if we should accept extra behavior on the already-complex cursor pagination. Not sure how to best take a judgement on this right now.

[cypreess]

@tomchristie I actually don't understand where is that complexity you mention. In pagination you have two concepts:

1. logic for finding page starting position - this can be anything, an offset, a cursor to value used for ordering, etc.
2. logic for saying how many elements to retrieve from start position - here just a single integer param of page_size

The only doubt I have here is about the role of offset_cutoff in whole story.

[joshmaker]

I hope this can be merged in at some point, because we really like this feature at The Atlantic. Ignoring the extensive unit tests, it's only a little bit more code and makes the CursorPaginator API work more like the other built in paginators.

[carltongibson]

OK, let's have this.

I was going to say "Needs Docs" but CursorPagination is already documented as taking a page_size param... 🙃

Thanks for the effort @cypreess!