Standardize legal headers to original LICENSE.md

[deepyaman]

Description

See #366

Development notes

make legal passes. Did some manual testing by including other directories like features (one of the files will fail due to having a non-ASCII apostrophe), one of the files in docs doesn't have a header.

The overall enhancements I made along the way:

• It's not really necessary to check the single LICENSE.md file against a string in tools/licenses_and_headers.py; in fact, the tool can use LICENSE.md as the header source rather than maintaining yet another file for it (this is where the two got out of sync).
• There's a mixture of writing generators and using non-lazy (hard-working?) methods like glob. The generator still gets fed directly to a list constructor, but c'est la vie.
• Prefer sys.exit in a script to exit (see https://stackoverflow.com/questions/6501121/difference-between-exit-and-sys-exit-in-python or myriad other SO answers).

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change and added my name to the list of supporting contributions in the RELEASE.md file
• Added tests to cover my changes

Notice

• I acknowledge and agree that, by checking this box and clicking "Submit Pull Request":

• I submit this contribution under the Apache 2.0 license and represent that I am entitled to do so on behalf of myself, my employer, or relevant third parties, as applicable.

• I certify that (a) this contribution is my original creation and / or (b) to the extent it is not my original creation, I am authorised to submit this contribution on behalf of the original creator(s) or their licensees.

• I certify that the use of this contribution as authorised by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[921kiyo]

Thanks!

[mzjp2]

LGTM!