Please refer to #2694 and #2670 for motivation and reasoning for
this change.

I've tried to follow best practice in inserting the copyright
headers. In other open source projects, not all files carry
the notice. For example documentation doesn't. I've followed
similar ground rules.

I did not change the php because there is a separate LICENSE
file there by Pixel Federation. We'll first need to notify
them our intent before changing anything there.

As for the presubmit check, it's going to be non-trivial
because of the number of exceptions, like file types,
directories and generated code. So, it will have to be
a separate project.