[v2] Add support for internationalization with TranslateErrors middleware

[rschef]

Add support for internationalization with TranslateErrors middleware:

• Rename HandleChangesetErrors middleware to TranslateErrors
• Implement internationalization using Gettext and default Crudry.Translator module
• Allow overriding default translator Crudry.Translator by adding a custom translator in Absinthe's context
• Allow overriding default language en by passing a custom locale in Absinthe's context
• Translate Ecto error messages using domain defined in Translator.errors_domain/0, which defaults to errors
• Translate Ecto schema fields using domain defined in Translator.schema_fields_domain/0, which defaults to schema_fields
• Add tests
• Improve modules docs

[gabrielpra1]

It looks like you're missing a test for when the translator does not define the expected functions
https://coveralls.io/builds/24744730/source?filename=lib%2Fmiddlewares%2Ftranslate_errors.ex#L126