Navigations between sections of a book in VitalSource happen in several
steps:

 1. The iframe for the previous chapter is removed
 2. An iframe is created for the new chapter. The initial load of this
    iframe contains a "blank" page with the chapter content in an
    encoded/encrypted form as hidden text in the page.
 3. The chapter content data is submitted to the VS backend via a form
    POST request, which returns the decoded content

Previously the Hypothesis client would sometimes detect the new iframe
after step 2 and inject the client before step 3 had completed. As a
result the client would be injected into the blank frame, then
immediately unloaded, and not re-loaded into the "real" content frame.

Make injection more reliable by:

 1. Checking whether the content frame is a "blank" frame or not before
    injecting the client.
 2. Listening for `load` events on the frame and re-injecting the client
    if the iframe is navigated