Add checkbox based multiselect util component

Takes an array of objects (name, checked, description) and turns it into
a list of checkboxes, one for each element

frontend-reactive/app/components/util/multiselect.hbs

@@ -0,0 +1,13 @@
+<div class={{@wrapper-class}}>
+  {{#each this.args.elements as |element|}}
+    <label class={{@label-class}}>
+      {{element.description}}
+      <Input
+        @type="checkbox"
+        name="{{element.name}}"
+        @checked={{element.checked}}
+        {{on "change" this.onChange}}
+      />
+    </label>
+  {{/each}}
+</div>