Allow manual refreshes to bypass phasing

[Stéphane Graber]

In the case of LXD, we primarily rely on phasing to avoid hammering and taking down the store.

As mentioned in https://bugs.launchpad.net/snapstore-server/+bug/1946029, we are doing monthly feature release which can have users be quite eager to switch to the new release.

With phasing enabled, it will take at least 24h before everyone gets the new release.
We've heard that new installs don't get phased which is great, but as a pre-installed snap, it's not doing us much good :)

Instead, it'd be great if there was a way to configure a snap so that phasing only applies to its background refreshes. That way those users who are just using it and aren't actively looking for the new release will get it eventually based on the phasing. But users who really want the new release can just run a manual "snap refresh lxd" to get it.

I seem to remember this pattern being somewhat common in other stores that have phasing, basically showing more recent stuff to those who actively hit the "check update" button.