[suggestion] provide a `Clear my data from the sync server` switch/button

[diracdeltas]

Per discussion in Slack, there should already be a way for clients to implicitly delete all sync data from the server, either when the last device is removed from the chain or when the main device is removed (see #201).

I think there should also be an explicit way for a client to tell Brave, "delete all my synced data on the server".

This would not delete local data on any of the clients.

It could be an additional switch in this menu for instance:

[bryan-snyder]

this is a great idea