Add button in dashboard to rebuild cache

[hugopeek]

Especially after clearing the custom Overview cache, large websites suffer a big performance hit until every page is revisited again. Far from ideal, if not unacceptable..

A rebuild cache button would activate a script that visits each page (preferrably in the background), so it will already be cached when the first visitor arrives. Unless the visit occurs between clear and reload of course, but there's not much we can do about that.

This will be one in the category: "it won't be pretty, but it'll do". Pointers:

https://www.cloudtec.ch/blog/web/2014/modx-cache-rebuild-refresh
http://bobsguides.com/refreshcache-tutorial.html
https://docs.modx.com/revolution/2.x/developing-in-modx/advanced-development/caching