Update to 1.9.0 ( https://github.com/eclipse-theia/theia/blob/v1.9.0/CHANGELOG.md#v190---16122020 )

Update extensions to newer version and to the URLs from openvsx instead of
github, since openvsx prevent maintainers to modify releases.

Also use buildout directly to download extensions, so that we verify signatures and have possibility to use cache

See merge request !873