[Admin] Add Ability to Change Namespace

[brianking]

More and more we are getting cases where owner want to transfer their extensions to a new namespace. I don’t believe this can be done currently, so what it means that they have to start from scratch and lose users who installed from the old namespace, as well as losing download count and other information. Would it be practical to add the ability to change namespaces?

Example request: https://github.com/EclipseFdn/open-vsx.org/issues/877

@amvanbaren It's possible to add the ability to change an extension's namespace in the admin dashboard. Basically, the server just has to change namespace_id in the extension table. Some additional error handling is needed before changing the namespace_id. The search index needs to be updated and caches invalidated if possible. All in all it's pretty doable