(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/ui2018/cssVars.ts

@@ -157,10 +157,12 @@ export const testId: TestId = makeTestId('test-');
 // Min width for normal screen layout (in px). Note: <768px is bootstrap's definition of small
 // screen (covers phones, including landscape, but not tablets).
 const mediumScreenWidth = 768;
+const smallScreenWidth = 576;   // Anything below this is extra-small (e.g. portrait phones).
 
 // Fractional width for max-query follows https://getbootstrap.com/docs/4.0/layout/overview/#responsive-breakpoints
 export const mediaSmall = `(max-width: ${mediumScreenWidth - 0.02}px)`;
 export const mediaNotSmall = `(min-width: ${mediumScreenWidth}px)`;
+export const mediaXSmall = `(max-width: ${smallScreenWidth - 0.02}px)`;
 
 export const mediaDeviceNotSmall = `(min-device-width: ${mediumScreenWidth}px)`;