Re-work pagination and add tests.

Here's a first version. This version doesn't include `next` or `previous` in the metadata if a number for page can't be found. This is only really expected to happen when `next` or `previous` are null for the first and last page. An alternative would be include `next` and `previous` in the metadata and set them to -1 for the first and last pages. What do you think?

I also still need to write user facing docs but I'm waiting for that to be setup.
What is the advantage between this and, say, returning `null`?

Then, you could arguably re-write the rest of extractMeta to look something like:

```js
metadata['next'] = extractPageNumber(payload.next);
metadata['previous'] = extractPageNumber(payload.previous):
```

Our interface would be that meta.next/previous will be null if they do not exist.