fix: import htmlSafe from @ember/template, not @ember/string (#992)

Author: ascudder

addon/components/ember-table/component.js

@@ -1,6 +1,6 @@
 import Component from '@ember/component';
 import { computed } from '@ember/object';
-import { htmlSafe } from '@ember/string';
+import { htmlSafe } from '@ember/template';
 import {
   setupTableStickyPolyfill,
   teardownTableStickyPolyfill,