Rename .object-list to .user-list

No issue
References todo comment in [users.scss](https://github.com/TryGhost/Ghost/blob/91c07e37bf5f6467ff65181581e8c7621033ab31/core/client/assets/sass/layouts/users.scss#L6-L7)

- Renames anything starting with `.object-list` to `.user-list`
- Adjusts tests to match
Merge pull request #4276 from PaulAdamDavis/object-list-users

Rename .object-list to .user-list