Convert Config entity to use "id" instead of "@group", refs T849

Summary:
Previously, the Config entity used `@group` instead of using `id` as the identifier.
This was inconsistent and resulted in some odd hacks to work around the behavior.
Cleaned up by injecting the group as `id` in the repository and fixing relevant tests.

Test Plan:
```
bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.acl.feature
bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.config.feature
bin/phpspec run spec/Ushahidi/Entity/ConfigSpec.php
```

Reviewers: vladimir, sheesh, aMoniker

Reviewed By: aMoniker

Subscribers: spaudanjo, rjmackay, srutto

Maniphest Tasks: T849

Differential Revision: https://phabricator.ushahidi.com/D472