(core) flesh out "View As" feature

Summary:
The users shown by the "View As" button are now drawn from more sources:
 * There are users the document is shared with. This has been rationalized, the behavior was somewhat erratic. If the user is not an owner of the document, the only user of this kind that will be listed is themselves.
 * There are users mentioned in any user attribute table keyed by Email. If name and access columns are present, those are respected, otherwise name is taken from email and access is set to "editors".
 * There are example users provided if there are not many other users available.

Test Plan: added and extended tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3045

app/client/components/DocComm.ts

@@ -56,6 +56,7 @@ export class DocComm extends Disposable implements ActiveDocAPI {
   public checkAclFormula = this._wrapMethod("checkAclFormula");
   public getAclResources = this._wrapMethod("getAclResources");
   public waitForInitialization = this._wrapMethod("waitForInitialization");
+  public getUsersForViewAs = this._wrapMethod("getUsersForViewAs");
 
   public changeUrlIdEmitter = this.autoDispose(new Emitter());