Adds AGPL 3.0 license

[JasonStoltz]

Summary

Kibana recently added a new license. EUI will likely need to do the same. This PR will remain in draft until we are able to confirm.

As we are adding an additional license option, I don't believe that this impacts any current users. I don't think semantically we need to consider this a break change.

More detail about the license change can be found here.

For reference, this is the PR for the last time we updated our license.

To review, please be sure to check commit by commit. There are a lot of updated files, but the majority of them are a bulk change to source file headers.

QA

• Ensure that the footer in our docs looks correct.
• Ensure readme links work
• Double check for any references to our old license
• Ensure that everything still builds and works with Kibana's license checker
• Ensure the emotion boilerplate example still works correctly

General checklist

• Release checklist
  • A changelog entry exists and is marked appropriately.

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 99c5d86

[JasonStoltz]

We are going to hold off on this change for now.