Removing need to keep around base dset for the header widget; now just saving what is shown -- the first n lines of the base dataset -- as a json, and loading if it's cached.

Removing any need for a dataframe in expander_general_stats; instead making sure to cache and load the small amount of details needed for this widget. Note I also moved around a couple functions -- same content, just moved -- so that it was easier for me to navigate through the code. I also pulled out a couple of sub-functions from larger functions, again to make the code easier to work with/understand, as well as helping to further modularize so we can limit what needs to be cached.

:tada: init