Provide a clean way to generate the base URL

[netomi]

There exists the UrlUtil class that generates URLs to be used by the app.

This class tries to deduce the base url from the current request context, which also means the class can only be used reliably within a request context, as otherwise it will return an invalid url.

Furthermore there is the setting ovsx.webui.url that is used in some places to override the deduced base url from the request.

This should be made clean by providing a

• ovsx.webuil.url
• ovsx.server.url

and use that values to create an app URL regardless of the request context.

By default the ovsx.server.url should default to the ovsx.webui.url, but could be overwritten in case api and ui are different (which is not the case for open-vsx.org right now).