Refactor the Link header to have a helper to generate them

[dstufft]

We're copy/pasting some hardcoded method for generating a Link url, ideally we should be able to simply modify an attribute on a Response to set this.

[dstufft]

I'm going to close this, I've started porting Warehouse over to Pyramid to ease the concerns of the custom framework-ish thing I've got going on here. That work is now on the master branch inside this repository. This is no longer going on in the new code base.