Add a way to go back to add a sync code

[bbondy]

I accidentally went into create a new sync account, and then had no way to go back to sync this device to another one. Please make a way to go back to that main screen and reset sync back to initial state if the user does the wrong selection like that.

cc @diracdeltas @diracdeltas

[diracdeltas]

i'll add a button to clear sync state completely

[bbondy]

++ thanks

[ayumi]

Related: brave/sync#51