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/controllers/contributors/edit.js

@@ -0,0 +1,18 @@
+import Controller from '@ember/controller';
+import { inject as service } from '@ember/service';
+
+export default Controller.extend({
+
+  kredits: service(),
+
+  actions: {
+
+    save (attributes) {
+      return this.kredits
+                 .updateContributor(this.model.id, attributes)
+                 .then(() => this.transitionToRoute('index'))
+    }
+
+  }
+
+});

app/controllers/contributors/new.js

@@ -1,21 +1,16 @@
 import Controller from '@ember/controller';
-import { alias } from '@ember/object/computed';
 import { inject as service } from '@ember/service';
 
 export default Controller.extend({
 
   kredits: service(),
 
-  contributors: alias('kredits.contributors'),
-
   actions: {
 
     save (contributor) {
-      return this.kredits.addContributor(contributor)
-        .then(contributor => {
-          this.transitionToRoute('index');
-          return contributor;
-        });
+      return this.kredits
+                 .addContributor(contributor)
+                 .then(() => this.transitionToRoute('index'))
     }
 
   }