Assign security advisory ID and creation date on publish

Problem/Motivation

Security advisories are currently copied to www.drupal.org just before release. It would be good to have the ability to draft them earlier.

Proposed resolution

Since advisories won’t be created in the same order they are published, the advisory ID should be set on publish.

• Do not require the advisory ID field.
• Replace ID default value setting with a message showing what the advisory ID will be set to on publish.
• When the advisory is being published, actually set the ID.

This makes the advisory ID field a good indicator of when an advisory is first published. (Very rarely an advisory might be temporarily unpublished and republished, that shouldn‘t reset the creation date.) When the advisory ID is being set on publish, also set the node creation date to the current request time.