Add abilities to storefront info

[jameswestman]

• Add reserve_custom_value_prefixes field, which allows us to block custom values by prefix and not just exact value. For example, we could reserve flathub:: so that apps can't upload appstream files with those custom keys.
• Allow a URL or custom value to be null, which removes it if it exists. This is important because otherwise, old keys may accumulate until the next time the app is uploaded.

[jameswestman]

cc @danielsilverstone-ct

[jameswestman]

Closing in favor of #82.