Disabling automatic refresh for snap from store

[Ads20000]

@MoHaG

Providing an option like this for snap publishers can help… (They can set it to 100% for critical security updates and quite low for risky feature updates, in order to prevent all devices from breaking at once) (For manual refreshes an option (enabled by default?) can be provided to update anyway to the latest release in the channel or stick to the normal rules)

Actually that’s an idea, phased updates like what Ubuntu does (rolling out first to 20% or so users, then 40%, then 60%, 80%, 100%, reverting to 0% if automated tests fail) should probably be supported.