Ensure Current.organization is set in REST API

What does this MR do and why?

In !158626 (merged), Current.organization is being set using Gitlab::Current::Organization. This works for web and GraphQL requests but not for Grape API (REST), because it is not using Rails ApplicationController.

This MR adds a before hook that will set Current.organization for the current request

Related to #474831 (closed)

MR acceptance checklist

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How to set up and validate locally

We can validate this change by creating a new Group using REST API. The validation of this change is not trivial, because the database has DEFAULT 1 for namespaces.organization_id column. So even without a Organization, the Group will be created. For the sake of testing, we remove the default 1:

• Create a Personal Access Token using http://gdk.test:3000/-/user_settings/personal_access_tokens
• Using gdk psql: alter table namespaces alter column organization_id drop default, alter column organization_id set not null;
• Enable feature flag using gdk rails c: Feature.enable(:require_organization)

on master branch, this will fail

• curl -XPOST --header "Content-Type: application/json" -d '{"name":"My New Group", "path":"my_new_group"}' --header "PRIVATE-TOKEN: <your token>" "http://gdk.test:3000/api/v4/groups"

on this branch, it will succeed and the group will be created

Don't forget to restore the database:

alter table namespaces alter column organization_id default 1, alter column organization_id set not null;