Indices Lack Scoped Uniqueness Constraints

[toastercup]

To ensure data integrity and performance, we need to:

• Ensure all relevant indexes are also unique
• Set up partial indexes so deleted_at records are not considered for uniqueness
• Set up partial indexes so records are scoped to their Tenant/Organization or ContentType (unsure how possible all of this is)

See:

• http://blog.bigbinary.com/2016/07/20/rails-5-adds-support-for-expression-indexes-for-postgresql.html
• https://jacopretorius.net/2017/04/validate-uniqueness-in-rails.html