(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/AccountWidget.ts

@@ -105,8 +105,9 @@ export class AccountWidget extends Disposable {
 
       // Show 'Organization Settings' when on a home page of a valid org.
       (!this._docPageModel && currentOrg && !currentOrg.owner ?
-        menuItem(() => manageUsers(currentOrg), 'Manage Team', testId('dm-org-access'),
-          dom.cls('disabled', !roles.canEditAccess(currentOrg.access))) :
+        menuItem(() => manageUsers(currentOrg),
+                 roles.canEditAccess(currentOrg.access) ? 'Manage Team' : 'Access Details',
+                 testId('dm-org-access')) :
         // Don't show on doc pages, or for personal orgs.
         null),