Fix PermissionError with multiple private tabs on Firefox

[noyainrain]

To reproduce:

1. Open Hello in private tab
2. Open Hello in another private tab
3. In first tab choose Edit user settings -> Save
4. Observe permission error notification

[noyainrain]

The behavior is caused by Firefox sharing cookies, but not localStorage between private tabs (see https://bugzilla.mozilla.org/show_bug.cgi?id=1453699 and https://bugzilla.mozilla.org/show_bug.cgi?id=1436231). It looks like this will be fixed with a new localStorage implementation.

For the steps outlined above, this is what happens behind the scenes:

1. (Tab 1) Cookie /, localStorage / -> Log-in as new user A -> Cookie A, localStorage A
2. (Tab 2) Cookie A, localStorage / -> Log-in as new user B -> Cookie B, localStorage B
   (Tab 1) -> Cookie B, localStorage A
3. (Tab 1) Edit user A with authentication B