(core) allow non-owners to remove themselves from sites/workspaces/docs

Summary:
For users who cannot otherwise change access to a resource, let
them remove themselves. Implemented via the standard endpoints
as a special exception that will process a request from a user
that would otherwise be denied, if the only contents of that
request are a removal of themselves.

Users who can change access are still not permitted to change their
own permissions or to remove themselves, as a precaution against
orphaning resources.

Test Plan: extended and updated tests

Reviewers: cyprien

Reviewed By: cyprien

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3367

app/client/ui/AppHeader.ts

@@ -37,7 +37,7 @@ export class AppHeader extends Disposable {
         activeEmail: user ? user.email : null,
         resourceType: 'organization',
         resourceId: org.id,
-        resource: org,
+        resource: org
       });
     };