Ensure only one current_organization and rename to instance_owner

[solebared]

Why

Discovered that every organization currently created is defaulted to is_instance_owner: true.
IIUC, only one org should be the instance owner.

Also renames current_organization because that name connotes something transient, eg current_user.

What

• Migration to change organizations.is_instance_owner default to false
• Model validation to ensure no two organizations can be instance owners
• Rename Organization.current_user -> .instance_owner
• Change Organization.instance_owner from a scope to a class method since it doesn't return a relation

Testing

• Added a model spec
• Manually tested org form

Next steps

• Disableis_instance_owner attribute on form and add a note. (instance owner has to be seeded)
• Add an issue to the roadmap to support multiple hosts using the same instance. Support multiple host organizations using the same deployed instance #930

Outstanding Questions, Concerns and Other Notes

It's quite possible i'm completely misunderstanding the intention of current_organization!
@maebeale, how does this align with your initial/current thinking?

Accessibility

This does make it harder for admins to change which org is the instance owner. If that becomes a needed feature, we can add a 'transfer ownership' feature.

Pre-Merge Checklist

• Security & accessibility have been considered
• Tests have been added, or an explanation has been given why the features cannot be tested
• Documentation and comments have been added to the codebase where required
• Entry added to CHANGELOG.md if appropriate
• Outstanding questions and concerns have been resolved
• Any next steps have been turned into Issues or Discussions as appropriate

[solebared]

Whelp, i just noticed #240, where @maebeale explicitly says this should not be renamed :)
Let's discuss!

[maebeale]

Hey! I'm ok with this!