Add AvoidDefaultOrganization cop to prevent .default_organization usage

What does this MR do and why?

Adding a new rubocop to prevent usages of Organizations::Organization.default_organization.
Using this method makes our migration to cells harder as there is no default organization on a new cell.
Instead, organizations should be derived from another model (i.e project, group etc), or looked up with Organization.find.

Once this is merged, I'll complete steps 3-4 from our documentation:

References

Relates to Add Rubocop to prevent new usages of Default Or... (#582629)

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