Gracefully handle lack of IndexedDB in Firefox Private browsing

[jeffgca]

Summary

There is no IndexedDB in Firefox Private Windows, but we need it, so we fail eg on loading drive.fission.codes

Problem

Firefox doesn't allow IndexedDB in Private Windows. They hopefully eventually will, the relevant Firefox bug is:

https://bugzilla.mozilla.org/show_bug.cgi?id=1639542

Impact

People who want to use drive in a private window, cannot currently. Drive works fine in Chrome's incognito mode.

Solution

When we feature-detect for IndexedDB the user experience on Drive currently is an infinite spinner, and as well a console message that doesn't give us much info. We should replace this with a link to documentation on browser compatibility that we can maintain outside this repo.

Detail

To Reproduce Steps to reproduce the behavior: 0. Open Firefox

1. Go to drive.fission.codes

Expected behavior

Drive should work as expected.

Screenshots

Desktop (please complete the following information):

• OS: macOS 10.15.latest
• Browser Firefox
• Version 88

Additional context

It remains to be seen whether Firefox will ever fix this bug, traffic seems stagnant despite apparently there being problems with Netflix:

( Now, Netflix's error is also not quite right, updating will not fix the issue. )