Update contributor profiles

* Adds a new page for updating profiles
* Refactors the add-contributor component to allow for updating
  existing contributors
* Adds input labels to the contributor form and improves placeholders
* Adds event handlers for all contract contributor changes and uses them
  for updating the UI

refs #122

app/templates/contributors/edit.hbs

@@ -0,0 +1,13 @@
+<main class="center-column">
+
+  <section id="add-contributor">
+    <header>
+      <h2>Update contributor profile</h2>
+    </header>
+
+    <div class="content">
+      {{add-contributor attributes=attributes save=(action "save")}}
+    </div>
+  </section>
+
+</main>