load statistics page faster
Currently all stats are loaded at once, they can be loaded via AJAX instead.
also, the statistics code is not in great shape, making it hard to add new stats if we like.
fixes #15919 - statistics page is loaded via AJAX
- refactor how statistics data is being called, now it should be trivial
to add additional charts or consume other data from it. It should make
it much easier to reuse statistics charts in other places
(e.g. dashboard) or unify how fact charts are done.
- remove unused JSON response from the statistics controller.