Refactor Team REX Standup Rota

[Jongmassey]

#674 introduced the concept of non-spreadsheet based RotaReporters and some improvements to how staff slack/Github usernames are handled. The Team REX standup rota could be refactored to use these new structures, for clarity and such that users are notified on Slack when they are on duty in the Rota.

[mikerkelly]

Following on from #674, would like to also address if quick:

• re-name the dependabot rota as it's no longer using dependabot...
• add a note in people.py about the consequences of adding to or reordering TEAM_REX.