Add pagination to CollectionsController show method

[timstallmann]

We're developing a theme where we'd like to show all collection items (paginated) via the collections/show route rather than via items/browse. It strikes me as strange that the show method on the collections controller doesn't currently set any of the correct variables for pagination to work -- happy to work on a PR to add this functionality in, but was there some reasoning there that I should know about?

[zerocrates]

I don't think there's any grand reasoning behind there not being the option of pagination here, other than simply that it's a "show" page, and pagination is typically reserved for the browse pages.

My only real concern would be that this doesn't interfere with directing people to the "real" full browse, at least in default themes.