migrate to GitHub Actions cla-assistant

[willnorris]

At least for the @twitter org, we've seen a lot of issues with cla-assistant.io not reporting on CLA status. CLA Assistant has a new (alpha quality) GitHub Actions based workflow that does not rely on the cla-assistant.io service at all, and instead stores CLA signatures in a JSON file in a git repo. We've experimented with this on the https://github.com/twitter/template repo, and it seems to work quite well.

There are two key downsides to this integration:

1. it must be configured on a per-repository basis, since that is the only way GitHub Actions works. Actions: Required workflows github/roadmap#52 tracks organization workflows, but there's no indication on what that will be available.
2. to fully work, it requires a personal access token with repo write permission, since that is required to rerun workflows. Without write permission on the repo doing CLA checking, a repo admin will need to rerun the CLA workflow after a new contributor has signed the CLA.

For #1, we will use a custom tool to replicate the workflow file across repositories. For #2, we will instruct repo admins to manually rerun the workflow. Fortunately, this is only necessary for brand new contributors, which doesn't happen too terribly often.

This issue will be used to track migration of all Twitter repositories to the new GitHub Actions based CLA workflow.