Add and verify working, canonical rel prev/next link tags

#5059 

#### Changes

* Added `template_preprocess_HOOK()` for `theme_pager_link()` to both os_basetheme and hwpi_basetheme.
* In both hooks, added code like below which adds a `<link />` tag to the head of the HTML document for the first occurance of each of: previous and next pager links.
* Consolidated logic into a single callback in `hook_process_HOOK()`, in a new inc file `pager.inc`