Detect and purge/flush cache plugins when popups or themes are saved

[danieliser]

Do it anytime we regenerate the static assets, so upgrades, settings saves etc.

This would cut down 10% or more support tickets.

From #547:

@danieliser @fpcorso - Maybe we should aim to just flush caches programmatically. We have the means, we can find other plugins that do it already and just copy what they do.

@fpcorso Doesn't each caching plugin manage that differently? Wouldn't flushing caches programmatically require us to utilize code to manage cache for each caching plugin?

@danieliser Correct, but that is why I suggested using another plugin that already offers that via the GPL to shortcut those concerns. There are a few plugins that do that, but Beaver Builder has it built in somewhere, a list of providers and callbacks for each is all it takes.