[cloud][CLOUD-73] Disallow site-admin access to organizations on Cloud

Created by: kopancek

Description

Disallow site-admin to access organizations on Cloud. Only members of the organization will have read-write access to organization settings, to view the organization fields (including name) and to view it's members.

Related items

https://sourcegraph.atlassian.net/browse/CLOUD-73

Screenshots

Testing locally

New behavior on Cloud

1. Run sg locally in dotcom mode: EXTSVC_CONFIG_ALLOW_EDITS=true sg start dotcom
2. Login as a site-admin
3. Create an organization
4. Logout and log in as a different site-admin (not a member of the organization)
5. Go to https://sourcegraph.test:3443/site-admin/organizations and verify that you cannot see the list of organizations
6. Try to access organization settings by going to https://sourcegraph.test:3443/organizations/PUT_ORG_NAME_HERE/settings and verify that you are shown an error

Old behavior applied when not on cloud

1. Run sg locally in enterprise mode: EXTSVC_CONFIG_ALLOW_EDITS=true sg start enterprise
2. Login as a site-admin
3. Go to https://sourcegraph.test:3443/site-admin/organizations and verify that you can see the list of all orgs defined on the instance
4. Try to access any organization settings that the site-admin is not a member off - this should work
5. Try to modify any organization settings that the site-admin is not a member off - this should work
6. Try to add a member to the organization directly, without creating an invite - this should work