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.

app/models/api_key.rb

@@ -1,6 +1,5 @@
 class ApiKey < ApplicationRecord
   belongs_to :user
-  validates_uniqueness_of :user_id
   validates_uniqueness_of :key
   has_secure_token :key