Plugin assets, external scripts and CSP overrides (#4260)

* Add assets for plugins
* Move plugin javascript and css into separate endpoints
* Allow loading external scripts
* Add csp overrides
* Only include enabled plugins
* Move URLMap to utils
* Use URLMap for assets
* Add documentation

internal/api/routes_custom.go

@@ -5,15 +5,14 @@ import (
 	"strings"
 
 	"github.com/go-chi/chi/v5"
-
-	"github.com/stashapp/stash/internal/manager/config"
+	"github.com/stashapp/stash/pkg/utils"
 )
 
 type customRoutes struct {
-	servedFolders config.URLMap
+	servedFolders utils.URLMap
 }
 
-func getCustomRoutes(servedFolders config.URLMap) chi.Router {
+func getCustomRoutes(servedFolders utils.URLMap) chi.Router {
 	return customRoutes{servedFolders: servedFolders}.Routes()
 }