Jetpack uses hardcoded wp-admin, potential problems with sites using custom paths.

**Problem:** Per [a user on the Jetpack support forums](https://wordpress.org/support/topic/plugin-doesnt-work-well-with-custom-wp-admin-path), Jetpack uses hardcoded `wp-admin` instead of `using get_admin_url();`. This will cause problems with sites that use a custom admin path (i.e. _not_ `wp-admin`).

**Solution:** use `using get_admin_url();` instead of hardcoded `wp-admin`.