MM-10367: rewrite subpath assets on startup (#8944)

Examine ServiceSettings.SiteURL on startup and rewrite assets
accordingly if not in a development environment.

Also export `mattermost config subpath` command to manually do same.

This accompanies a webapp PR to use the updated `root.html` to define
the necessary webpack asset path for dynamically loading assets.

web/static.go

@@ -18,6 +18,8 @@ import (
 
 func (w *Web) InitStatic() {
 	if *w.App.Config().ServiceSettings.WebserverMode != "disabled" {
+		UpdateAssetsSubpathFromConfig(w.App.Config())
+
 		staticDir, _ := utils.FindDir(model.CLIENT_DIR)
 		mlog.Debug(fmt.Sprintf("Using client directory at %v", staticDir))