Improve the Last Modified timestamps pn web page footers

Split the existing webwml_ctime() function into two: webwml_built_time() and webwml_modified_time().

In footer.wml, switch the lastmodified tag to use webwml_modified_time() and add a new lastbuilt tag using webwml_built_time(). Then display both in the standard footer text for each page.

Also added new PO entry for "Last Built", will need translating.