Allow plugin slug changes during approval-stage

[dd32]

First let me say: NO, we don't want to support plugins changing their slugs. This is super-complicated and out-of-scope of this ticket.

Plugins can currently have their slug changed prior to being approved, but once approved the team has no ability to change the slug. This is because it's at this point where the SVN directory is created, SVN access is granted, etc.

It would be useful if there was a tool that allowed a plugin reviewer to force-change a plugin slug in that approved stage, before the author commits code. This is sometimes needed as authors don't attempt to let the reviewer know their desired slug until too late, or, something happens on our end that causes the slug not to be set properly.

ie, it'd be a tool that works in the below 'approved' post_status:

• pending: New plugin submission, slug can be changed at will by reviewer.
• approved: Plugin submission approved, slug can no longer be changed.
• publish: Plugin author has committed code, slug changes are never permitted.

[dd32]

In 13729:

Plugin Directory: Allow plugin slugs to be renamed during the approval stage.

This allows plugin reviewers to change the slug immediately after the plugin is approved, but before the author commits any code.

Fixes #6878.