SSO administration (ExternalServices)

Rolling pull request

# Puavo

- [x] Rename existing ExternalService classes to LdapService
- [x] Add new ExternalService model for SSO
- [x] Create script to add new external applications
- [x] Add external application activation views for organisation
- [x] Add external application activation views for school
- [x] Translations for the views

# Rest LDAP

- [x] Check services against the trusted attribute
- [x] Check services against `School#puavoActiceService` attribute
- [x] Check services against `LdapOrganisation#puavoActiveService`  attribute
- [x] Display nice error message for user if service is not active
- [x] Separate services by path prefix

Hit merge when all boxes are checked