Change terraform_states.locked_by_user_id foreign key to SET NULL

What does this MR do?

Fixes an edge case where a user could not be deleted if they
held a lock (in the Terraform sense, not a database lock) on a
Terraform state, as it would violate the foreign key constraint.

Migrations

== 20210305002016 UpdateTerraformStatesLockedByUserIdForeignKey: reverting ====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:terraform_states)
   -> 0.0035s
-- execute("ALTER TABLE terraform_states\nADD CONSTRAINT fk_rails_558901b030\nFOREIGN KEY (locked_by_user_id)\nREFERENCES users (id)\n\nNOT VALID;\n")
   -> 0.0041s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- execute("ALTER TABLE terraform_states VALIDATE CONSTRAINT fk_rails_558901b030;")
   -> 0.0052s
-- execute("RESET ALL")
   -> 0.0008s
-- remove_foreign_key(:terraform_states, :users, {:name=>"fk_558901b030"})
   -> 0.0047s
== 20210305002016 UpdateTerraformStatesLockedByUserIdForeignKey: reverted (0.0339s)

== 20210305002016 UpdateTerraformStatesLockedByUserIdForeignKey: migrating ====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:terraform_states)
   -> 0.0040s
-- execute("ALTER TABLE terraform_states\nADD CONSTRAINT fk_558901b030\nFOREIGN KEY (locked_by_user_id)\nREFERENCES users (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0022s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE terraform_states VALIDATE CONSTRAINT fk_558901b030;")
   -> 0.0035s
-- execute("RESET ALL")
   -> 0.0010s
-- remove_foreign_key(:terraform_states, :users, {:name=>"fk_rails_558901b030"})
   -> 0.0038s
== 20210305002016 UpdateTerraformStatesLockedByUserIdForeignKey: migrated (0.0246s)

Does this MR meet the acceptance criteria?

Conformity

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