Introduce gitlab_main_org as an alias of gitlab_main_cell

We will use gitlab_main_org GitLab Schema to refer to the tables that contain Organization data. Such as projects, issues, namespaces, ...

Most of these tables allow sharding key for Organization Isolation

As part of this issue you need to:

• Add gitlab_main_org GitLab Schema as an alias for gitlab_main_cell. Make sure to allow cross joins and cross database modification with gitlab_main_cell tables as well. Because they might still be added while this MR is being merged.
• Add an rspec test that prevents newly introduced tables from belonging to gitlab_main_cell starting from next milestone.