Automation rules: support for updating version slug

[agjohnson]

This might be at the limit of what editable versions were meant for, but it would be really nice to be able to have automation rules add version slugs so that users could have a rule like:

• 4\..* matching versions follow semantic versioning and the latest gets custom slug 4.0
• 3\.3\..* matching versions follow semantic versioning and the latest gets custom slug 3.3
• 2\..* matching versions follow semantic versioning and the latest gets custom slug 2.0

The issue is maybe in removing the previous custom slug however, ie:

• Before, version 2.2.2 has custom slug 2.0
• After, version 2.3.0 has custom slug 2.0 and version 2.2.2 has custom slug reverted/removed