[Self-hosting] “Premium” support in iOS app should be enabled

Now that premium purchases and tiers work in self-hosting, the iOS mobile app should be updated with the necessary functionality. For example:

• The premium badge should show
• The label for premium should be the instance-defined label
• The in-app premium screen should be enabled and allow gift code redemptions

Since it’s iOS I understand that actually purchasing premium (assuming a hoster has enabled stripe) through the app is likely not feasible, but an instruction to log in from the desktop app would likely work

Enable tiered (free, premium) support on a self-hosted instance