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.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57156