MM-10370: serve subpath (#8968)

* factor out GetSubpathFromConfig

* mv web/subpath.go to utils/subpath.go

* serve up web, api and ws on /subpath if configured

* pass config to utils.RenderWeb(App)?Error

This allows the methods to extract the configured subpath and redirect
to the appropriate `/subpath/error` handler.

* ensure GetSubpathFromConfig returns trailing slashes deterministically

* fix error 404 handling

* redirect /subpath to /subpath/

This is necessary for the static handler to match, otherwise none of the
registered routes find anything. This also makes it no longer necessary
to add trailing slashes in the root router.

app/server.go

@@ -29,10 +29,17 @@ import (
 type Server struct {
 	Store           store.Store
 	WebSocketRouter *WebSocketRouter
-	Router          *mux.Router
-	Server          *http.Server
-	ListenAddr      *net.TCPAddr
-	RateLimiter     *RateLimiter
+
+	// RootRouter is the starting point for all HTTP requests to the server.
+	RootRouter *mux.Router
+
+	// Router is the starting point for all web, api4 and ws requests to the server. It differs
+	// from RootRouter only if the SiteURL contains a /subpath.
+	Router *mux.Router
+
+	Server      *http.Server
+	ListenAddr  *net.TCPAddr
+	RateLimiter *RateLimiter
 
 	didFinishListen chan struct{}
 }
@@ -99,7 +106,7 @@ func redirectHTTPToHTTPS(w http.ResponseWriter, r *http.Request) {
 func (a *App) StartServer() error {
 	mlog.Info("Starting Server...")
 
-	var handler http.Handler = &CorsWrapper{a.Config, a.Srv.Router}
+	var handler http.Handler = &CorsWrapper{a.Config, a.Srv.RootRouter}
 
 	if *a.Config().RateLimitSettings.Enable {
 		mlog.Info("RateLimiter is enabled")