Downcase aliased OAuth2 callback providers
Users may specify an OAuth2 callback with a custom name, such as AWSCognito, but Rails will reject this with the following message:
import/AWSCognito' is not a supported controller name. This can
lead to potential routing problems. See
http://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use
To avoid these errors, we can just downcase all the provider names. Note that this will make it impossible to specify a duplicate name with different cases.