(core) Admin Panel and InstallAdmin class to identify installation admins.

Summary:
- Add InstallAdmin class to identify users who can manage Grist installation.

  This is overridable by different Grist flavors (e.g. different in SaaS).
  It generalizes previous logic used to decide who can control Activation
  settings (e.g. enable telemetry).

- Implement a basic Admin Panel at /admin, and move items previously in the
  "Support Grist" page into the "Support Grist" section of the Admin Panel.

- Replace "Support Grist" menu items with "Admin Panel" and show only to admins.

- Add "Support Grist" links to Github sponsorship to user-account menu.

- Add "Support Grist" button to top-bar, which
  - for admins, replaces the previous "Contribute" button and reopens the "Support Grist / opt-in to telemetry" nudge (unchanged)
  - for everyone else, links to Github sponsorship
  - in either case, user can dismiss it.

Test Plan: Shuffled some test cases between Support Grist and the new Admin Panel, and added some new cases.

Reviewers: jarek, paulfitz

Reviewed By: jarek, paulfitz

Differential Revision: https://phab.getgrist.com/D4194

app/client/components/Forms/FormView.ts

@@ -496,7 +496,7 @@ export class FormView extends Disposable {
         async (dontShowAgain) => {
           await this._publishForm();
           if (dontShowAgain) {
-            this.gristDoc.appModel.dismissedPopup('publishForm').set(true);
+            this.gristDoc.appModel.dismissPopup('publishForm', true);
           }
         },
         {
@@ -588,7 +588,7 @@ export class FormView extends Disposable {
         async (dontShowAgain) => {
           await this._unpublishForm();
           if (dontShowAgain) {
-            this.gristDoc.appModel.dismissedPopup('unpublishForm').set(true);
+            this.gristDoc.appModel.dismissPopup('unpublishForm', true);
           }
         },
         {