(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/lib/localStorageObs.ts

@@ -48,11 +48,15 @@ function createStorage(): Storage {
 
 /**
  * Helper to create a boolean observable whose state is stored in localStorage.
+ *
+ * Optionally, a default value of true will make the observable start off as true. Note that the
+ * same default value should be used for an observable every time it's created.
  */
-export function localStorageBoolObs(key: string): Observable<boolean> {
+export function localStorageBoolObs(key: string, defValue = false): Observable<boolean> {
   const store = getStorage();
-  const obs = Observable.create(null, Boolean(store.getItem(key)));
-  obs.addListener((val) => val ? store.setItem(key, 'true') : store.removeItem(key));
+  const storedNegation = defValue ? 'false' : 'true';
+  const obs = Observable.create(null, store.getItem(key) === storedNegation ? !defValue : defValue);
+  obs.addListener((val) => val === defValue ? store.removeItem(key) : store.setItem(key, storedNegation));
   return obs;
 }