[BUG] Disable automatic sync to avoid creating duplicate highlights

**Is your feature request related to a problem? Please describe.**
If you have multiple instances of Remnote open (e.g. app and browser, or Remnote open on two devices) then you will have duplicate highlights or duplicate parent books.

**Describe the solution you'd like**
Ideally if the highlight or the book already exists then the plugin shouldn't create duplicates, but if this is not feasible when multiple instances of Remnote are running then there should be an option to disable automatic syncing so the user can run the plugin when they need to import the highlights only.