Fix EZP-22128: Change inline license and copyright info to refer to files

*Status: Call for feedback*

This is a proposal for how to do it deal with copyright and license headers as proposed in https://jira.ez.no/browse/EZP-22128.

The issue proposes that we should adopt for a copyright and license header approach similar to Symfony does, see: [php file](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Config/ConfigCache.php) and [LICENSE file](https://github.com/symfony/symfony/blob/master/LICENSE). The reason for this is to simplify maintenance of the code a bit and make it simpler to deal with several licenses in the future in regards to delivering our source code (also enterprise) over composer.

Deviations taken in this PR:
- Opted to create a COPYRIGHT file to avoid confusion between the copyright of the license iself (as is the case in our [GPL LICENSE file](https://github.com/ezsystems/ezpublish-community/blob/master/LICENSE)) and the source code
- Opted to keep on using \@license php doc to simplify replacement and it is afaik supported by doc generators adding some value.


Command used (NB: OSX flavor of sed):
```find . -name *.php | xargs sed -i "" -e "s#@license .*\$#@license For copyright and license information see COPYRIGHT and LICENSE files distributed with this source code.#;s#@copyright .*\$#@copyright Copyright (C) eZ Systems AS. All rights reserved.#"```