Remove "Under Revision" status

JIRA story https://osraav.atlassian.net/browse/OSRA-112

- remove 'Under Revision' Status from seeds.rb
- change methods in initializer.rb
- change calls to initializer methods in Sponsor, Partner & Orphan models
- change relevant specs
- remove references to 'Under Revision' status from sponsor.feature (it didn't break anything, but for the sake of completeness and consistency)
Good point. Both this method & `default_sponsorship_status_to_unsponsored` are highly unlikely to be shared with any other models