Leadership: Add Core Committer

Adding a new Core Committer

When a new core committer is added to the team, there are a number of places they need to be included, and things that need to be udpated, as follows:

1. Update the leadership page backdropcms.org/leadership so that everybody knows that they are a core committer.

2. Add the new person to the backdrop/backdrop repository, so that they can have commit access to core code.

3. Add the new person to the Backdrop Security Team located at https://github.com/orgs/backdrop-ops/teams/security, so that they can help coordinate security releases, and contribute to security fixes.

4. Add the new person to the backdrop-ops/leadership repository, so that they can have access to all the issues where PMC + Core committers collaborate.

5. Add the person's preferred email address to the core commiter group email forwarding, set up at NameCheap.

6. (Until we choose a better chat tool that integrates with GitHub) a) Manually add the new person to the private "Core committers" chat channel. b) Manually add the new person to the private "Leadership" chat channel.