(core) Make mobile the default mode.

Summary:
- Make unsupported browser warning into an unobtrusive one-liner, similar in
  style to notifications.
- Move browser warning details into a support page, linked from "Learn more" link.
- Show different mobile and desktop warnings.
- Once dismissed, remember dismissal for a year rather than just for the session.
- Turn the Sign-In button (for anon users) into a menu (for the sake of exposing
  the Toggle Mobile Mode option)
- Improve styling of HomeIntro screens when on small screen.
- Flip the default for setting mobile viewport to true

Test Plan: Added minor unittest for localStorageBoolObs; fixed other affected tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2738

app/client/ui/HomeIntro.ts

@@ -6,7 +6,7 @@ import {examples} from 'app/client/ui/ExampleInfo';
 import {createDocAndOpen, importDocAndOpen} from 'app/client/ui/HomeLeftPane';
 import {buildPinnedDoc} from 'app/client/ui/PinnedDocs';
 import {bigBasicButton} from 'app/client/ui2018/buttons';
-import {colors, testId} from 'app/client/ui2018/cssVars';
+import {colors, mediaXSmall, testId} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {cssLink} from 'app/client/ui2018/links';
 import {commonUrls} from 'app/common/gristUrls';
@@ -127,6 +127,12 @@ function buildExampleItem(doc: Document, home: HomeModel, view: 'list'|'icons')
 const cssIntroSplit = styled(css.docBlock, `
   display: flex;
   align-items: center;
+
+  @media ${mediaXSmall} {
+    & {
+      display: block;
+    }
+  }
 `);
 
 const cssIntroLeft = styled('div', `
@@ -134,6 +140,7 @@ const cssIntroLeft = styled('div', `
   overflow: hidden;
   max-height: 150px;
   text-align: center;
+  margin: 32px 0;
 `);
 
 const cssIntroRight = styled('div', `