curl check and acquia_migrate_migrate_form_submit

- Check for curl in acquia_migrate_settings_form_validate, I didn't have it installed on my local machine so cloudapi login failed.
- Added "migrate code" and "migrate files" buttons to acquia_migrate_migrate_form
- Added acquia_migrate_migrate_form_submit with switch for both buttons, fetching remote environment id and local docroot path then returning in DSM.