Revoke admin privileges

[crawfoal]

Description

Admins can revoke admin privileges using the minus button next to the person's email on the admin list.

Developer Checklist

• [x] I have read and followed the contribution guidelines.
• [x] All specs and linters pass (including Code Climate)
• [x] I have followed the style of nearby code
• [x] The build on Semaphore is successful
• [x] I have added tests for my changes and these tests would fail without my changes.
• [x] I have gone through my changes by hand, either in the browser or terminal, which ever makes sense
• [x] I have internationalized my changes

Maintainer Checklist

• [ ] Run migrations (if necessary) in production after deployment
• [ ] Review issues and update labels