FEATURE: Multiple embeddable hosts

- Also refactors two site settings components into one, with tests

app/assets/javascripts/admin/templates/components/value-list.hbs

@@ -0,0 +1,18 @@
+{{#if collection}}
+  <div class='values'>
+    {{#each collection as |value|}}
+      <div class='value'>
+        {{d-button action="removeValue"
+                   actionParam=value
+                   icon="times"
+                   class="btn-small no-text"}}
+        {{value}}
+      </div>
+    {{/each}}
+  </div>
+{{/if}}
+
+<div class='input'>
+  {{text-field value=newValue placeholderKey=addKey}}
+  {{d-button action="addValue" icon="plus" class="btn-primary btn-small no-text" disabled=inputInvalid}}
+</div>