Don't show the view switcher until the first batch of annotations has
been received. (This first batch will be all the annotations, unless
there are more than 200 annotations of the page.)

This is an attempt to reduce "popping" caused by the annotation switcher
suddenly changing a brief time after first render, when annotations are
loaded:

https://github.com/hypothesis/product-backlog/issues/327#issuecomment-312657968