api keys: allow users to have multiple API keys.

This is useful if you have multiple programs and want to give them
different API keys, or if you want to rotate keys for a single program.

db/migrate/20210214095121_remove_uniqueness_constraint_from_api_keys.rb

@@ -0,0 +1,6 @@
+class RemoveUniquenessConstraintFromApiKeys < ActiveRecord::Migration[6.1]
+  def change
+    remove_index :api_keys, :user_id, unique: true
+    add_index :api_keys, :user_id, unique: false
+  end
+end