Pull request creation and link in editor UI

**Is your feature request related to a problem? Please describe.**
We use editorial workflow to allow contributors to add new content. We use pull requests to review content and either request changes or merge the content. With the current UI, this workflow is tricky for a few reasons:
- There's no link between the CMS and the pull request that it generates.
- Contributors save a draft, not realizing they've created a PR. Moderators manually indicate which PRs are ready for review by adding comments.
- Contributors see a "publish" button in the CMS and try to click it. If the `master` branch is protected, clicking this button causes an error. Otherwise, clicking this button causes the PR to be merged without review.

**Describe the solution you'd like**
I'd like to see a configuration flag that replaces the "publish" button with a link to create or view a pull request. In this mode, "draft" and "ready" status would be used as an indicator of whether a PR has been created.

Mockup of "draft" status:

Mockup of "ready" status:

**Describe alternatives you've considered**

- We've tried protecting master to disable publishing, but doing so breaks media uploads (#1344).
- We added a feature request for showing draft/ready status as a label in a pull request (#1617). Many users don't seem to realize that they're creating a PR by saving a draft, though.
- There's a proposal for an `allow_publish` flag (#1298) to disable publishing within the CMS. This feature would help prevent users from publishing content without review, but it wouldn't clarify which PRs are ready for review or show the user that the next steps happen in a PR.