Bookmarks XHR should not be fetching unless user is signed in

**Problem**
- with incognito window, go to: https://archive.org/details/goody
- open console -> you see a 401 return for bookmarks call
- spin up BookReader's demo server -> go to Internet Archive `http://127.0.0.1:8080/BookReaderDemo/demo-internetarchive.html?ocaid=theworksofplato01platiala`
- open console -> you see a 404 return for bookmarks call

**Expectation**

- if user is not signed in, do not fire XHR call
- if user is signed in, request bookmarks appropriately=

**Proposal & Constraints**

check `ia-bookmarks.js` -> note that we pass in a property `displayMode` - https://github.com/internetarchive/bookreader/blob/master/src/BookNavigator/bookmarks/ia-bookmarks.js#L100

let's try:
- changing `displayMode` default from `bookmarks` to `login`
- ensure that the initial API call does not get fired until `displayMode` flips to `login`

This can be handled in the lit element's life cycle `updated`, perhaps.

**Stakeholders**

@dualcnhq @iisa