db: add uniqueness constraints

Enforce uniqueness constraints at the database level, on:
- Invites,
- Procedures,
- Individuals.

These are the one less likely to have duplicates. A follow-up
PR will contains similar migrations, but more likely to have
existing duplicates that need to be removed.

db/migrate/20210427120002_add_unique_index_to_individuals.rb

@@ -0,0 +1,14 @@
+class AddUniqueIndexToIndividuals < ActiveRecord::Migration[6.1]
+  include Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    delete_duplicates :individuals, [:dossier_id]
+    add_concurrent_index :individuals, [:dossier_id], unique: true
+  end
+
+  def down
+    remove_index :individuals, [:dossier_id]
+  end
+end