Move away from generic `camptix_stats` option

[pkevan]

There are a few problem associated with a generic stats options - it's unclear how much it is used by site users (through the shortcode), but overall it would make sense to split the value between a few options, any of those which are used regularly, or updated regularly are probably best not autoloaded etc.

Ideally, the payment stats can be grouped together, and the form stats another, then perhaps the last modified can be another.

Whichever way, by splitting these stats we should be able to working with them easier.

Thoughts?

[pkevan]

related: https://github.com/WordPress/wordcamp.org/issues/1417

[pkevan]

https://github.com/WordPress/wordcamp.org/pull/1424 was merged, and data migrated to new option for all previous stats.