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.

app/controllers/graphql_controller.rb

@@ -29,7 +29,7 @@ class GraphqlController < ApplicationController
   private
 
     def consul_schema
-      api_types  = GraphQL::ApiTypesCreator.create(API_TYPE_DEFINITIONS)
+      api_types  = GraphQL::ApiTypesCreator.create
       query_type = GraphQL::QueryTypeCreator.create(api_types)
 
       GraphQL::Schema.define do