Allow switching of organizations

We need to allow users to switch organizations and then keep track of which organization the user is active in. See #417778 (closed) for switcher designs.

Engineering guide

• Add current_organization_name and current_organization_id to lib/gitlab/gon_helper.rb
• Using Disclosure create a Vue component that renders the available organizations and current organization in app/assets/javascripts/super_sidebar/components.
• Check feature flag and render component in app/assets/javascripts/super_sidebar/components/user_bar.vue#L151
• Use app/assets/javascripts/organizations/index/graphql/organizations.query.graphql to get available organizations when disclosure is opened.
• Current organization will be a link to that organization
• Switching organizations will be a link to the current page but in the context of that organization. For example, /dashboard/groups for organization Foo bar would be /-/organizations/foo-bar/dashboard/groups