Add a confirmation step/warning message when a maintainer makes a stable release

To paraphrase semver.org- if your software is going to be used in production, has a stable api that users can depend on, or if you're concerning yourself with backwards compatibility, you should probably make a stable release. In the Drupal community, we also recommend that the maintainer be vetted to demonstrate a grasp of good coding and security practices.

Conversely, if your project should not yet be used in production, does not have a stable api, and will not maintain backwards compatibility - you should probably not make a stable release.

To encourage good release management behavior, we'll be adding a confirmation step and warning message when a maintainer tries to make a stable release. This step will remind them what a stable release should be, and strongly encourage them to apply for vetted status so they can opt the project in to receive security advisory coverage once they create that stable release.

Production outage: No
Change affects: Drupal.org Projects
Deployment schedule: week of 2017-03-13
Backout plan: revert or fix in place