Add codeowners file

[nathanmsmith]

yay code review!

[dustinnewman]

@nathunsmitty what is @dailybruin/internal-sites-editors? is that a github org?

[nathanmsmith]

it's a GitHub team in the db org that is made up of you and me – I'd like you to at least be requested for review on all PRs in Internal Tools even if you aren't the one approving most of them. The reason why it's a team is so that when we have a new internal tools person next year, we can just add them to the team and leave it without having to change every codeowners file across all DB repos

[dustinnewman]

@nathunsmitty LGTM!