(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/DocMenuCss.ts

@@ -1,5 +1,5 @@
 import {transientInput} from 'app/client/ui/transientInput';
-import {colors, vars} from 'app/client/ui2018/cssVars';
+import {colors, mediaSmall, vars} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {styled} from 'grainjs';
 
@@ -15,6 +15,11 @@ export const docList = styled('div', `
     display: block;
     height: 64px;
   }
+  @media ${mediaSmall} {
+    & {
+      padding: 32px 24px 24px 24px;
+    }
+  }
 `);
 
 export const docListHeader = styled('div', `
@@ -216,6 +221,12 @@ export const prefSelectors = styled('div', `
   right: 64px;
   display: flex;
   align-items: center;
+
+  @media ${mediaSmall} {
+    & {
+      right: 24px;
+    }
+  }
 `);
 
 export const sortSelector = styled('div', `
@@ -236,4 +247,9 @@ export const sortSelector = styled('div', `
     box-shadow: none;
     background-color: ${colors.mediumGrey};
   }
+  @media ${mediaSmall} {
+    & {
+      margin-right: 0;
+    }
+  }
 `);