annotation template: allow browsing through sections (in long documents)

[daliboris]

The sample annotations are very short, but when working with long documents (divided into parts), only the first part is displayed on the page and it is not possible to go to the next (or previous) part.

To enable this navigation, you can add the following elements to the annotate.html template in the header toolbar:

<pb-navigation emit="transcription" keyboard="left" subscribe="transcription" direction="backward" unit="page" rendition="invisible">
  <paper-icon-button icon="icons:chevron-left"/>
</pb-navigation>
<pb-navigation emit="transcription" keyboard="right" subscribe="transcription" direction="forward" unit="page" rendition="invisible">
  <paper-icon-button icon="icons:chevron-right"/>
</pb-navigation>