Reverts the grunt-contrib-copy implementation of the images and webfonts tasks

Restores the original task implementations. For vanilla-config users, nothing changes. For users who have overridden the webfonts or images configs using the old style (`img.root`, `img.app`, `img.vendor`, `webfonts.root`, or `webfonts.vendor`), nothing changes.

For users who downloaded v0.15.* in the last couple weeks AND have overridden the grunt-contrib-copy style configuration (by modifying `images.dev/dist.files` or `webfonts.dev/dist.files` in the application config), they will receive a warning message and the task will fail (unless forced).

```
Warning: The 'images' task implementation has been reverted. It currently does not
support the grunt-contrib-copy style configuration, which has been detected in
your setup. You will likely need to configure 'img.app', 'img.vendor' and/or
'img.root' in 'config/files.js'. Use --force to continue.
```
or
```
Warning: The 'webfonts' task implementation has been reverted. It currently does not
support the grunt-contrib-copy style configuration, which has been detected in
your setup. You will likely need to configure 'webfonts.vendor' and/or
'webfonts.root' in 'config/files.js'. Use --force to continue.
```