Remove navigator.locks from user preferences request

[matthew-white]

As of #1024, Frontend sends requests to update user preferences from the propagate() method of the UserPreferences class. Currently, each request is wrapped in navigator.locks.request(), but after discussion, @brontolosone and I think that isn't strictly necessary. This issue is to update propagate(), removing navigator.locks.request(). I have an idea about how to do so, so I'll start by giving that a try, then sharing my idea with @brontolosone once it's ready.