Fix pan with not behaving correctly with very large (>2^32) article numbers

This was basically down to using strtoul instead of strtoull.

However, after fixing that and looking around I found a bunch of places where different variants of string->integer were used. So I refactored everything into two classes, Article_Number and Article_Count. At some point some error checking needs to go into the two new constructors, but I imagine it'll be a while before a new group has anywhere near 2^64 articles.

While debugging things, I discovered that 'last N days' for large groups didn't work, which was a side effect of a previous fix for dealing with interrupting large group updates in !22 (merged)

PS Sorry for the size of this. Creating new types was the only way I could be sure I'd covered all the places where the wrong conversion was being used.

Fixes #62 (closed)