[Feature Planning] Opt-in sync for settings and whitelist

[mwsundberg]

Hoisted to a separate issue from #47.

Overview:
Users with browsers signed in to a Firefox Account will be able to sync extension settings across their devices. This is a browser native feature that will not send data to any external servers (except Mozilla's, which was already happening due to being signed in).
Settings sync will be off by default.

Seeking feedback:
Specifics of sync behavior need clarifying:

• The simplest option is to read and write directly to storage.sync when sync is enabled on a device.
• uBlock Origin syncs by pushing and pulling changes from storage.sync to storage.local instead of directly using it. This might be the best approach, but it takes two actions to have a settings change on one device affect another (push change to sync to share, then pull to local on other device to use)

Potential Foot-Guns:
Care must be taken to not overwrite anyone's existing settings on extension updates or change/wipe settings without confirmation including information of what's going to be changed.

Stretch goals:

• Fine grained control over which settings are synced (only sync toggle settings not whitelists, more relevant as features are added)
• Stage settings changes for review (locally before syncing and when pulling settings synced by other devices) with highlighted differences (eg. added x,y,z to whitelist, disabled notifications)

[mwsundberg]

Can't change the issue labels but this would be an Enhancement. I'm currently working on #46 so I won't get to this for a little while. Plus it's a pretty low priority feature while we just have just 2 booleans and 1 string array as the sum total of our settings to hypothetically sync lol