refs #5091, #6612
- unify getNextUrl & getPrevUrl into getPaginatedUrl
- ensure that it can generate a prev, next or exact page no url
- ensure that it can figure out the base url
- use the same code from the page_url helper
- refactor the tests to ensure there's 100% coverage
Following on from #6612, this ensures that pagination always works regardless of whether the channel is default or custom
