Prompt author for custom commit message on publish

**Problem**
Users have no chance to give their commits a custom message.
A simple real-world use case is the following:
Large non-tech teams using the netlify-cms need some way to tie their content edits to a ticket.
The commit message templates beta feature from [https://www.netlifycms.org/docs/beta-features/#commit-message-templates](https://www.netlifycms.org/docs/beta-features/#commit-message-templates) is limited.

**Solution**
When hitting publish, a beautiful modal would prompt the user for a chance to input a commit message. In the use case mentioned above, the user could finally input their ticket number.

The option to prompt for a commit message could be simply toggled on or off via a config backend parameter:

```
backend:
commit_messages:
prompt: true | false
```

**Additional context**

The UI would visually be something along the lines of: