(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/DocMenuCss.ts

@@ -216,17 +216,9 @@ export const spinner = styled('div', `
 `);
 
 export const prefSelectors = styled('div', `
-  position: absolute;
-  top: 32px;
-  right: 64px;
+  float: right;
   display: flex;
   align-items: center;
-
-  @media ${mediaSmall} {
-    & {
-      right: 24px;
-    }
-  }
 `);
 
 export const sortSelector = styled('div', `