Improved commit message docs in contributing guide
Improved commit message docs in contributing guide

- `refs/closes` can be confusing to those who are not familiar with GitHub when we actually mean "use refs or fixes (or closes)"
- we generally don't want to use emojis for alpha features because adding it to the release notes is unnecessary until the feature is GA
TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost