Add the file for the default configuration values and its validation schema

The 2.0.x branch has the config/install/graph_mail.settings.yml and the config/schema/graph_mail.schema_yml files, which are the files containing the default values for the configuration object used by the module, and the schema used to validate those settings.

These files need to be back-ported to the 1.0.x branch, as those files are necessary for any module with configuration objects. Having the config/install/graph_mail.settings.yml file since the beginning would have avoided the code that checks if a configuration value is set, as with that file all the configuration values are set to their default values.