Use custom element names for adder toolbar (#3276)

Using custom element names for the adder's container is a simple trick
to reduce the likelihood of the adder's styling being affected by CSS
selectors in the containing page, such as example.com's `div` selector.

h/static/scripts/annotator/adder.html

-<div class="annotator-adder">
-  <div class="annotator-adder-actions">
+<hypothesis-adder class="annotator-adder">
+    <hypothesis-adder-actions class="annotator-adder-actions">
       <button class="annotator-adder-actions__button h-icon-annotate" data-action="comment">
         <span class="annotator-adder-actions__label" data-action="comment">Annotate</span>
       </button>
       <button class="annotator-adder-actions__button h-icon-highlight" data-action="highlight">
         <span class="annotator-adder-actions__label" data-action="highlight">Highlight</span>
       </button>
-  </div>
-</div>
+    </hypothesis-adder-actions>
+</hypothesis-adder>