(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/common/UserAPI.ts

@@ -151,6 +151,7 @@ export interface PermissionDelta {
 }
 
 export interface PermissionData {
+  personal?: boolean;
   maxInheritedRole?: roles.BasicRole|null;
   users: UserAccessData[];
 }