Truly disable all updates via WP admin UI

Based on [this article](https://make.wordpress.org/core/2013/10/25/the-definitive-guide-to-disabling-auto-updates-in-wordpress-3-7/) about completely disabling automatic and/or UI-driven updates in WP (which is what we want in Bedrock AFAIK) and on my own experiments:

`DISALLOW_FILE_MODS` implicitly includes `AUTOMATIC_UPDATER_DISABLED` and `DISALLOW_FILE_EDIT`, plus:

* It hides the update notifications in the admin bar.
* It hides the "Updates" sub-menu in the "Dashboard" admin menu.
* It hides the "Update Available" messages in "Appearance -> Themes" and "Plugins -> Installed Plugins".

I verified in the WP source that `DISALLOW_FILE_MODS` actually includes `AUTOMATIC_UPDATER_DISABLED` and `DISALLOW_FILE_EDIT`, and it is confirmed respectively [here](https://github.com/WordPress/WordPress/blob/master/wp-admin/includes/class-wp-upgrader.php#L2374) and [here](https://github.com/WordPress/WordPress/blob/master/wp-includes/capabilities.php#L1258).