Handle index.html correctly in custom served folders (#3168)

* getStringMapString return nil if key not found
* Refactor custom routes. Handle /index.html

internal/api/routes_custom.go

@@ -0,0 +1,35 @@
+package api
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/go-chi/chi"
+	"github.com/stashapp/stash/internal/manager/config"
+)
+
+type customRoutes struct {
+	servedFolders config.URLMap
+}
+
+func (rs customRoutes) Routes() chi.Router {
+	r := chi.NewRouter()
+
+	r.HandleFunc("/*", func(w http.ResponseWriter, r *http.Request) {
+		r.URL.Path = strings.Replace(r.URL.Path, "/custom", "", 1)
+
+		// http.FileServer redirects to / if the path ends with index.html
+		r.URL.Path = strings.TrimSuffix(r.URL.Path, "/index.html")
+
+		// map the path to the applicable filesystem location
+		var dir string
+		r.URL.Path, dir = rs.servedFolders.GetFilesystemLocation(r.URL.Path)
+		if dir != "" {
+			http.FileServer(http.Dir(dir)).ServeHTTP(w, r)
+		} else {
+			http.NotFound(w, r)
+		}
+	})
+
+	return r
+}