Fixed upgrades for paid members (#15760)
Fixed upgrades for paid members (#15760)

closes https://github.com/TryGhost/Team/issues/2202

Some parts of the codebase were not using the isSameCurrency helper
which meant that we were incorrectly filtering out tiers from the
upgrade screen. Tiers used to *usually* have a lowercased currency
property, but they now _always_ have an uppercased.
TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost