composer.json - Add all bower assets via composer-downloads-plugin

composer.json - Add all bower assets via composer-downloads-plugin

Before
------

* JS and CSS libraries and downloaded via `bower`.
* `bower` is deprecated.
* `bower` is slow.
* `bower` requires installing NodeJS
* `bower` requries installing Bower

After
-----

* `composer install` downloads the `bower_components` folder
* `composer` has a better cache
* `composer` is fast
* You can setup a dev build with fewer dependencies.

Comments
--------

There are many, many composer plugins which can be referenced when managing
assets. This particular one has an important distinction:

* It does not require root-level package configuration.
* It produces a file-structure that closely matches the current one.
* It works just as well as whether 'civicrm-core' is used as a root-package
  or as an dependency.