indicate loading when upgrade/downgrade is happening

[chase-moskal]

• currently, cancelling or renewing a tier will show a loading indicator (see the subscriptions submodel for that logic)
• what we want to do, is add a loading indicator for upgrade/downgrades
• the important thing is, we do NOT want to show loading indicator the whole time a checkout popup is in progress
• probably the simplest way, is for the purchase function in that submodel to accept a showLoadingIndicator boolean
  • and the component can provide that boolean based on each switch-case for the purchase scenario