Update allowed tags according to AMP specification

[westonruter]

[Edited] Refer to the latest acceptance criteria.

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It appears that the amp_wp_build.py script hasn't been run since it was introduced at the end of 2016. Since that time, the changes to class-amp-allowed-tags-generated.php appear to have been done manually. There have been 95 commits to the spec since the end of 2016, and these changes need to be comprehensively added to the plugin.

I understand the amp_wp_build.py script has to be updated to account for some changes to the spec and to the amphtml project. For example, a dependency has been removed which has to be reverted or restored in some way: ampproject/amphtml#10830

Additionally, the spec changes need to be more regularly applied, such as via Travis CI cron, if not done manually.

[ThierryA]

Acceptance criteria:

1. Conduct a Discovery to fully determine how the existing script is working.
2. Based on the findings of the Discovery, determine the best way to fix the components/functions of the script that are either broken or not working as intended.
3. More details to come post Discovery...

Additional acceptance criteria:
4. Create the new allowed tags file, class-amp-allowed-tags-generated.php
5. Open a pull request to the plugin repo with that new file, updated PHPUnit tests, and this commit to prevent an error in accessing an undefined property and update the extensions/paths 6. Submit a pull request to amphtml to revert the deletion of validator_gen_md.py, making this commit.
6. Create a bin/update-allowed-tags script that runs everything, including composer etc.