Go over caching one more time

[creesch]

Because it seems like I introduced some oddities last time I reworked caching.

• This isn't actually consumed on the front so cache objects aren't cleared for existing pages. I am not sure if that is a good or a bad thing though, certainly something that needs to figure out.
• Some cached data is set on init if the cache object is empty, like moddedsubs. However if that is the first tab to open (and thus init of the background page) this function simply clears everything.

[creesch]

A little update... seems like some things got conflated with each other... https://github.com/toolbox-team/reddit-moderator-toolbox/blob/master/extension/data/tbcore.js#L1469