Fixed slug-based navigation in collections UI

refs https://github.com/TryGhost/Team/issues/3169

- When navigating through collections slugs did not display correctly returning "undefined" in the navigation. It was easier to fix the bug than working around it.

ghost/admin/app/components/collections/collection-form.hbs

@@ -14,6 +14,16 @@
                 </span>
             </GhFormGroup>
 
+            <GhFormGroup @errors={{@collection.errors}} @hasValidated={{@collection.hasValidated}} @property="slug" class="mr2 flex-auto">
+                <label for="collection-slug">Slug</label>
+                <input type="text" class="gh-input" id="collection-slug" name="slug" value={{@collection.slug}} {{on "input" (pick "target.value"
+                    (fn this.setCollectionProperty "slug" ))}} {{on "blur" (fn this.validateCollectionProperty "slug" )}}
+                    data-test-input="collection-slug" />
+                <span class="error">
+                    <GhErrorMessage @errors={{@collection.errors}} @property="slug" />
+                </span>
+            </GhFormGroup>
+
             <GhFormGroup class="no-margin" @errors={{@collection.errors}} @hasValidated={{@collection.hasValidated}}
                 @property="description">
                 <label for="collection-description">Description</label>

ghost/admin/app/router.js

@@ -51,8 +51,8 @@ Router.map(function () {
     this.route('tag', {path: '/tags/:tag_slug'});
 
     this.route('collections');
-    this.route('collection.new', {path: '/collection/new'});
+    this.route('collection.new', {path: '/collections/new'});
-    this.route('collection', {path: '/collection/:collection_slug'});
+    this.route('collection', {path: '/collections/:collection_slug'});
 
     this.route('settings-x');
     this.route('settings');

ghost/admin/app/routes/collection.js

@@ -27,7 +27,7 @@ export default class CollectionRoute extends AuthenticatedRoute {
     }
 
     serialize(collection) {
-        return {collection_slug: collection.get('collection')};
+        return {collection_slug: collection.get('slug')};
     }
 
     @action