feat(broadcasts): add admin broadcast page and user opt-out switch

Admin page /admin/broadcasts to write, test and send broadcasts in both
languages; confirms the recipient count before send. History shows the
counts. A new switch in settings lets users opt out of platform mails;
account mails always come. Subject and body are plain text only.

src/components/AppHeader.test.jsx

@@ -55,6 +55,7 @@ describe('AppHeader', () => {
 
         expect(await screen.findByText('NAV.ADMIN')).toBeTruthy();
         expect(screen.getByText('NAV.ADMIN_NEWS')).toBeTruthy();
+        expect(screen.getByText('NAV.ADMIN_BROADCASTS')).toBeTruthy();
         expect(screen.getByText('NAV.VIEW_AS.MENU')).toBeTruthy();
         expect(screen.queryByText('NAV.VIEW_AS.BADGE')).toBeNull();
         expect(screen.queryByLabelText('NAV.PENDING_REQUESTS:0')).toBeNull();
@@ -84,6 +85,7 @@ describe('AppHeader', () => {
         openMenu();
         expect(await screen.findByText('NAV.VIEW_AS.MENU')).toBeTruthy();
         expect(screen.queryByText('NAV.ADMIN')).toBeNull();
+        expect(screen.queryByText('NAV.ADMIN_BROADCASTS')).toBeNull();
     });
 
     it('offers nothing of that to a user', async () => {
@@ -96,6 +98,7 @@ describe('AppHeader', () => {
         expect(screen.getByText('NAV.WHATS_NEW')).toBeTruthy();
         expect(screen.queryByText('NAV.ADMIN')).toBeNull();
         expect(screen.queryByText('NAV.ADMIN_NEWS')).toBeNull();
+        expect(screen.queryByText('NAV.ADMIN_BROADCASTS')).toBeNull();
         expect(screen.queryByText('NAV.VIEW_AS.MENU')).toBeNull();
     });
 });