Make it possible to disable currency conversion functionality

For RHEL we have customers that have auditing requirements that forbid non-allowlisted things from going out on the network without the user initiating it.

gnome-calculator goes out on the network to download currency conversion data.

This patchset provides a way for the admin to opt out of that behavior by setting a refresh interval of 0 in settings.

It also hides the conversion UI in that case, since showing the conversion UI when the conversion data is out of date isn't that useful.