Add categories validations

This MR adds validation for:

1. stages.yml:
   1. categories exist in categories.yml
   2. feature labels are not being added here (should be in categories.yml)
   3. fixes errors, moving non-existent categories to maintained_categories
2. features.yml:
   1. categories exist in categories.yml
   2. fixes errors, updating the category if able, removing or marking as unknown if not

The MR also adds maintained_categories as a place to search for a category when looking up the group.