Update openedx-webhooks to use team membership to apply labels

[itsjeyd]

Relates to:

• Discovery: How to get a global list of CCs to use for automation purposes #149
• Create prototype app to list all core contributors for a repo #148

Axim is already moving towards a github team for each repo. And as of early Feb 2025, CC data that is available to openedx-webhooks includes a new column called "Is Core Contributor" which provides global/repo-agnostic info about someone's CC status. (Ref)

So update the openedx-webhooks codebase to apply the core contributor label to a PR if its author is a CC.