Fix an issue where multiple Hypothesis sidebar (and possibly notebook, profile
etc.) iframes could get created when saving a local snapshot of a web page where
Hypothesis was loaded. This would lead to one of the sidebars not functioning
because the host frame only allows one sidebar to connect.

Aside from there being multiple sidebars, there are other problems in such an
environment, such as the URL not matching the original page URL.  Hence this
commit handles this scenario by detecting it when the annotator bundle loads,
removing the existing `<hypothesis-sidebar>` element and then aborting startup.

The end result for the user is that if they save a snapshot of a page with
Hypothesis loaded and later reload it, the visible highlights will still appear
in the page but other Hypothesis UI elements will not be visible.

The scenario described here applies in Chrome when saving a snapshot with the
"Web Page, Complete" file type. If the user instead selects "Web Page, HTML
only" or "Web Page, single file" they will get different results, but those
were already not problematic because no JS was executed.

Fixes https://github.com/hypothesis/client/issues/5827