Reinstate search pagination, with fixes

This reinstates the search pagination, which was added in #655 (and temporarily reverted in #658).

There are a couple of fixes added here:
- we force the `start` parameter to be a string when we build the request to Rummager
- we offset the result number used in click tracking by the current value of `start` (so that a click on the first result on page two would be result 51, not 1)

Please take a look at the individual commits over the whole branch, as the entire pagination feature is shown in the full changes.