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/routes/contributors/edit.js

@@ -0,0 +1,29 @@
+import { inject as service } from '@ember/service';
+import Route from '@ember/routing/route';
+import { alias } from '@ember/object/computed';
+
+export default Route.extend({
+
+  kredits: service(),
+  contributors: alias('kredits.contributors'),
+
+  model(params) {
+    return this.kredits.contributors.findBy('id', params.id);
+  },
+
+  setupController (controller, model) {
+    this._super(controller, model);
+
+    controller.set('attributes', {
+      account: model.account,
+      name: model.name,
+      kind: model.kind,
+      url: model.url,
+      github_username: model.github_username,
+      github_uid: model.github_uid,
+      gitea_username: model.gitea_username,
+      wiki_username: model.wiki_username
+    });
+  }
+
+});