Remove `postdate` and `fulldate` columns from `cpanstats` table

[preaction]

These two date/time-ish fields are duplicates of created (#24), and so do not need to exist. But before they can be removed, all code that depends on them must be updated to use created instead:

• APIs/pages that filter or sort by date should use created
• APIs/pages that display the date should use created with the appropriate (compatible) format

API compatibility is paramount and breakage is not allowed. HTML page compatibility is nice to have: If it's an ordeal to maintain exact compatibility on any HTML, we won't. Creating new, backwards-compatible replacement APIs and pages is preferred to spending egregious amounts of time maintaining legacy APIs and pages.