Update approval process & inbox UI [Do not merge]

Supersedes https://github.com/unikent/programmes-plant/pull/328

- [ ] Add new inbox UI (one button plus review page with full diff view)
- [ ] Support for basic diff from live restored
- [ ] Reject action removed
- [ ] Refactor diff code
- [ ] Use new "awaiting_review", removing need for extra status (start removing status)
- [ ] Basic review -> process working