Simplify updates

We need to make updates/upgrades easier. To do this, we should be able to identify one platform as a successor or 'upgrade path' for another platform. This'd mean we only need to have an 'upgrade' task, instead of migrating to some arbitrary platform. The upgrade task would just call migrate under the covers, of course. But the UI would be greatly simplified.

This is essentially how we already operate in Aegir. A new platform is created, and probably through naming conventions, designated as the target for upgrades from previous platforms. I usually build platforms with the date as part of the name, to simplify this as much as possible. So Rune 2015-08-21, for example, would be a newer version of Rune 2015-07-18. I've also seen version tags used (kplatforms_3.2).

The problem, to begin with, is just that we're using a 'migrate' task to do updates. 'Migrate' describes the underlying process well, but it's not at all intuitive to end users. Next, migrate provides a list of all available platforms, which is fine, in and of itself. But it means that the person triggering the migration has to know (or at least be actively aware) of more than they really should.

It seems like the Relation module would provide us with an API we could use to define such relationships between platforms. We could then use that in the 'upgrade' task to figure out the target platform. We might want to add a 'create upgrade path' task to platforms that would essentially open the platform creation form, but pre-populate the source platform. #906936: Allow creation of a platform using an existing platform's information would probably be relevant here too.