app: Consolidate and correct references to app ID and config keys

- Make Application::APP_ID lowercase.
- Replace hardcoded 'SCIMServiceProvider' references with Application::APP_ID.
- Replace config key 'ExternalId' with 'externalId' to match SCIM data.

Example SQL to make corresponding changes in the DB:

update oc_preferences set appid='scimserviceprovider' where appid='SCIMServiceProvider';
update oc_preferences set configkey='externalId' where lower(appid)='scimserviceprovider' and configkey='ExternalId';