Change button tag to span for the minimal onboarding look.

The new note button in the onboarding tutorial shouldn't be a button because it isn't clickable.
It is there to teach the user what to look for to create a new note.
Turn the button element into a span with the same styling as the
new note button which appears in the 'Page Notes' tab.

See hypothesis/product-backlog#351
and https://github.com/hypothesis/client/pull/580#discussion_r151372544

src/sidebar/templates/sidebar-tutorial.html

@@ -60,9 +60,9 @@
     <li class="sidebar-tutorial__list-item sidebar-tutorial__list-item--is-theme-clean">
       <div class="sidebar-tutorial__list-item-content sidebar-tutorial__list-item-content--is-theme-clean">
         Create page level notes
-        <button class="sidebar-tutorial__list-item-new-note-btn" h-branding="ctaBackgroundColor">
+        <span class="sidebar-tutorial__list-item-new-note-btn" h-branding="ctaBackgroundColor">
           + New note
-        </button>
+        </span>
       </div>
     </li>
     <li class="sidebar-tutorial__list-item sidebar-tutorial__list-item--is-theme-clean">

src/styles/sidebar-tutorial.scss

@@ -83,6 +83,7 @@
   font-weight: 500;
   text-align: center;
   margin-left: 2px;
+  padding: 2px 5px;
 }
 
 .sidebar-tutorial__list-item-drop-down {