#267: jump to top/bottom buttons for long pastes and the editor

Fixed-position Top/Bottom pills appear only when content exceeds 2x
viewport height (window scroll on paste view, textarea scroll on /new).
New static/jump.js drives them; markup added to paste.html and new.html.

internal/web/templates/paste.html

@@ -55,8 +55,13 @@
     <div class="code" id="code"><div class="gutter" id="gutter">{{.Gutter}}</div><div class="codebody" id="codebody">{{.ContentHTML}}</div></div>
   </div>
   {{end}}
+  <div class="jumpnav hidden" id="jumpnav">
+    <button type="button" class="btn jump-btn" data-jump="top">Top</button>
+    <button type="button" class="btn jump-btn" data-jump="bottom">Bottom</button>
+  </div>
 </div>
 <input type="hidden" id="raw-content" value="{{.ContentAttr}}">
 <script src="/static/paste.js" defer data-paste-id="{{.ID}}"></script>
 <script src="/static/paste-lines.js" defer></script>
+<script src="/static/jump.js" defer></script>
 {{template "foot" .}}