Move preview_markdown.js to behaviors

app/assets/javascripts/behaviors/index.js

@@ -8,7 +8,7 @@ import installGlEmojiElement from './gl_emoji';
 import './quick_submit';
 import './requires_input';
 import './toggler_behavior';
-import '../preview_markdown';
+import './preview_markdown';
 installGlEmojiElement();
 initCopyAsGFM();

app/assets/javascripts/dropzone_input.js

 import $ from 'jquery';
 import Dropzone from 'dropzone';
 import _ from 'underscore';
-import './preview_markdown';
+import './behaviors/preview_markdown';
 import csrf from './lib/utils/csrf';
 import axios from './lib/utils/axios_utils';