Don't access the database in after_initialize

Rails 5.2 crashes in the `db:create` task because it tries to run the
`after_initialize` block before the database is created.

The easiest way to solve it is to move the code out of the initializer
and calculate the API type definitions on demand. Note results are still
cached using a class instance variable (not to be confused with a class
variable), and so once definitions are obtained, they will remain
constant until the application is restarted, even in the development
environment.

lib/graph_ql/api_types_creator.rb

@@ -10,7 +10,7 @@ module GraphQL
       string: GraphQL::STRING_TYPE
     }.freeze
 
-    def self.create(api_types_definitions)
+    def self.create
       created_types = {}
       api_types_definitions.each do |model, info|
         create_type(model, info[:fields], created_types)
@@ -18,6 +18,10 @@ module GraphQL
       created_types
     end
 
+    def self.api_types_definitions
+      @api_types_definitions ||= parse_api_config_file(YAML.load_file(Rails.root.join("config/api.yml")))
+    end
+
     def self.type_kind(type)
       if SCALAR_TYPES[type]
         :scalar