Extract EE specific files/lines for app/assets/javascripts

We have the following files containing EE specific code. We should move them to ee/

• app/assets/javascripts/api.js
• app/assets/javascripts/gfm_auto_complete.js
• app/assets/javascripts/gl_dropdown.js
• app/assets/javascripts/issuable_form.js
• app/assets/javascripts/main.js
• app/assets/javascripts/members.js
• app/assets/javascripts/milestone_select.js