Filter and sort roles when saving config

Problem/Motivation

There are currently a couple problems with the format of the exported configuration:

1. Roles are not sorted This may create unnecessary diffs when committing the file into configuration
2. Unused roles are saved into the configuration Because this is just a simple configuration and not a config entity, this is not such a big issue, but it's still not nice
3. Role names are duplicated Not a real big issue either, but would still be nice to fix

Proposed resolution

I think instead of just saving the raw form values to configuration, it would be better to save:

$values = array_keys(array_filter($form_values));
sort($values);

Remaining tasks

User interface changes

API changes

Data model changes