Determine Current.organization on requests

This is a continuation of the efforts started in #437541 (closed).

The current solution is:

1. Map Routable paths back to an organization. e.g. /gitlab-org/gitlab/issues/443829 -> default org.
2. When user is authenticated, use current_user.organizations.first. Routable path will override to reflect accurate HTTP 403/404/etc scenarios. (#464667 (closed))
3. Else, default organization.
   • When path has a uniqueness claim, the router will direct to the correct cell so this should work. E.g /some-username.
   • When path is ambiguous, this will not necessarily return a correct result. Maybe this is enough for Cells 1.0.

Obsolete issue description within.

In this issue we'll focus on adding the routed organization_id calculated from the route supplied by the router service described in #437541 (comment 1785691864) and !144811 (comment 1784126192)

1. Router returns HTTP 307 with added X-GL-ORG-ID: 123 header

flowchart TD
  browser -- gitlab.com/some group --> router
  router -- gitlab.com/some group --> classify
  classify -- status: 307; header: X-GL-ORG-ID: 123 --> router
  router -- status: 307; header: X-GL-ORG-ID: 123 --> browser

2. Browser makes a new request with added X-GL-ORG-ID: 123 header

flowchart TD
  browser -- gitlab.com/some group; header: X-GL-ORG-ID: 123 --> router
  router -- header: X-GL-ORG-ID: 123 --> cell_2_middleware
  cell_2_middleware -- Current.organization = 123 --> rails_stack

---

• Is possible for the classify service to do in the redirect(307) and can we decide on the header name(X-GL-ORG-ID for example) that will hold this info?

Proposal

• Determine if the above design is feasible and determine the header name. /cc @ayufan
• Add the logic to the current organization middleware to use this info, if provided to set the Current.organization if the initial HTTP_ header is not provided in lib/gitlab/middleware/organizations/current.rb