CakePHP - use composer to install

[JedMeister]

CakePHP recommends installation via composer so we should probably do that...

I have included the turnkey-composer overlay for v16.1 for users who wish to use composer to manage the existing install, but IMO we should look at moving the install method (to composer) for v17.0.

Also, currently conf.d/downloads uses a custom curl command to get the latest version. As we'll be changing install method, we may not need to work out the latest version (e.g. IIRC composer can install versions like latestandstable). However, if for some reason we do need explicit version info, we should leverage gh_releases` (from common) to determine the version (rather than custom curl).