(core) allow a doc owner to test access as a different user

Summary:
This adds back-end support for query parameters `aclAsUser_` and
`aclAsUserId_` which, when either is present, direct Grist to
process granular access control rules from the point of view
of that user (specified by email or id respectively).

Some front end support is added, in the form of a tag that
shows up when in this mode, and a way to cancel the mode.
No friendly way to initiate the mode is offered yet.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2704

app/gen-server/lib/HomeDBManager.ts

@@ -359,7 +359,7 @@ export class HomeDBManager extends EventEmitter {
   }
 
   public getUser(userId: number): Promise<User|undefined> {
-    return User.findOne(userId);
+    return User.findOne(userId, {relations: ["logins"]});
   }
 
   public async getFullUser(userId: number): Promise<FullUser> {