Allow to deactivate extension versions

[spoenemann]

Details on the transition of open-vsx.org to the Eclipse Foundation will be published very soon. This transition will bring new requirements to publishers (signing an agreement, choosing a license). We need a way to deactivate extensions of publishers who do not meet these requirements after a grace period, and to reactivate them after they have fulfilled the requirements. These changes must be enabled via application.yml configuration and will affect only the public instance running at open-vsx.org, not any self-hosted instances.

Proposed changes:

• Implement a procedure that finds all extension publishers who have not signed the Publisher Agreement. Set the active flag of corresponding extensions to false.
• Invoke the above procedure once after the grace period of the open-vsx.org transition has elapsed, either with a POST request or on application startup.
• When a publisher signs the Publisher Agreement, set the active flag of already published extensions to true.