Continue implementing widgets caching.

#6043 

This PR adds caching to the widgets from the list in https://github.com/openscholar/openscholar/pull/6151. 

When rendering via `os_boxes_widget_submit()` we render outside of the context. We should avoid caching this rendering, and I wasn't able to find a better solution than to use a global variable to do so.

I encountered problems due to this rendering couple of times, and I think this should be totally removed or replaced. Meanwhile, workaround applied.