Sync configuration between browsers

[GentlemanHal]

It would be good to be able to sync configuration between different browsers.

This would make it super simple to run Nevergreen on multiple browsers, such as different TVs around the office or on local developers machines.

Backup and restoring from a Gist (#119) allows users to do this, but copying the Gist ID is a little tedious as they can be up to 32 characters.

[GentlemanHal]

Playing #271 would keep configuration up to date remotely, so it might be good enough for this feature to just periodically pull the configuration down from the remote location, e.g. nightly.

This would require the user to make one copy of Nevergreen the "primary" that all updates happened on to avoid trashing configuration, but that is probably acceptable 🤔