Put a license notice to each file

[luixxiul]

Playbook Configuration: irrelevant

Matrix Server: irrelevant

Ansible: irrelevant

Problem description:
https://www.gnu.org/licenses/gpl-faq.en.html#NoticeInSourceFile says:

You should put a notice at the start of each source file, stating what license it carries, in order to avoid risk of the code's getting disconnected from its license. If your repository's README says that source file is under the GNU GPL, what happens if someone copies that file to another program? That other context may not show what the file's license is. It may appear to have some other license, or no license at all (which would make the code nonfree).

Adding a copyright notice and a license notice at the start of each source file is easy and makes such confusion unlikely.

I believe it would be appreciated by contributors if they are given a chance to add their names to the source code (regardless of the number of those who would like to do so actually), as I will.

Additional context
As an extensive contributor to https://github.com/element-hq/element-web/blame/develop/res/css/views/rooms/_EventTile.pcss (for styling of one of the most important components of Element Web), I regret that I have not pushed a request to put my name on it while I was working on it.

[spantaleev]

A CREDITS file (like the Linux kernel's CREDITS file) may be another option for adding credits which does not pollute files with metadata.

It doesn't help with the "license being close to the source code file" though, so the other one may be useful as well (although at the cost of polluting files).

[luixxiul]

One of the reasons why putting a license notice to each file is worth doing looks to be that it should make it easier to transplant files to another repository like MASH. Some of the existing files in docs/ directory of this project could be re-used over there.

[luixxiul]

This can be a good reference: cryptpad/cryptpad#1300 and cryptpad/cryptpad#1149 (comment). Also: https://reuse.readthedocs.io/en/stable/readme.html