Inform user when a draft is already in progress

**- Do you want to request a *feature* or report a *bug*?**
Feature

**- What is the current behavior?**
When editorial workflow is enabled, a user can open an existing file for editing without realizing another draft edit is already in progress. On the backend, both edits will be added to the same branch, and merged in the same PR. This can result in a user publishing more changes than expected.

**- What is the desired behavior?**
I could see a few improvements that could be useful here:
- In the collection list, add a tag to items currently in the editorial workflow, with their current status.
- When a user opens an existing file with an existing draft edit, show a message letting them know this.
- When the message appears to say a draft is already in progress, give the user the option to add their changes to that draft, or create a new one (with a new branch name) - potential for merge conflicts certainly goes up with this one, though.