(core) Add viewport meta tag conditionally, and show a toggle for it on small devices.

Summary:
- Enable narrow-screen layout for home page
- Clean up margins/spacing on small-screen home page
- Use "<768" as small-screen condition rather than "<=768".
- Include meta-viewport tag conditionally, off by default.
- Include "Toggle Mobile Mode" option in AccountMenu to toggle it on.
- In a test, add an after() clause to restore window size even when test fails

Test Plan: Only tested manually on iPhone (Safari & FF).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: cyprien

Differential Revision: https://phab.getgrist.com/D2708

app/client/ui/App.ts

@@ -10,6 +10,7 @@ import * as koUtil from 'app/client/lib/koUtil';
 import {reportError, TopAppModel, TopAppModelImpl} from 'app/client/models/AppModel';
 import {setUpErrorHandling} from 'app/client/models/errors';
 import {createAppUI} from 'app/client/ui/AppUI';
+import {addViewportTag} from 'app/client/ui/viewport';
 import {attachCssRootVars} from 'app/client/ui2018/cssVars';
 import {BaseAPI} from 'app/common/BaseAPI';
 import {DisposableWithEvents} from 'app/common/DisposableWithEvents';
@@ -154,6 +155,7 @@ export class App extends DisposableWithEvents {
 
     // Add the cssRootVars class to enable the variables in cssVars.
     attachCssRootVars(this.topAppModel.productFlavor);
+    addViewportTag();
     this.autoDispose(createAppUI(this.topAppModel, this));
   }