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

pkg/utils/urlmap_test.go

@@ -0,0 +1,70 @@
+package utils
+
+import (
+	"testing"
+)
+
+func TestURLMap_GetFilesystemLocation(t *testing.T) {
+	// create the URLMap
+	urlMap := make(URLMap)
+	urlMap["/"] = "root"
+	urlMap["/foo"] = "bar"
+
+	empty := make(URLMap)
+	var nilMap URLMap
+
+	tests := []struct {
+		name       string
+		urlMap     URLMap
+		url        string
+		wantNewURL string
+		wantFsPath string
+	}{
+		{
+			name:       "simple",
+			urlMap:     urlMap,
+			url:        "/foo/bar",
+			wantNewURL: "/bar",
+			wantFsPath: "bar",
+		},
+		{
+			name:       "root",
+			urlMap:     urlMap,
+			url:        "/baz",
+			wantNewURL: "/baz",
+			wantFsPath: "root",
+		},
+		{
+			name:       "root",
+			urlMap:     urlMap,
+			url:        "/baz",
+			wantNewURL: "/baz",
+			wantFsPath: "root",
+		},
+		{
+			name:       "empty",
+			urlMap:     empty,
+			url:        "/xyz",
+			wantNewURL: "/xyz",
+			wantFsPath: "",
+		},
+		{
+			name:       "nil",
+			urlMap:     nilMap,
+			url:        "/xyz",
+			wantNewURL: "/xyz",
+			wantFsPath: "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotNewURL, gotFsPath := tt.urlMap.GetFilesystemLocation(tt.url)
+			if gotNewURL != tt.wantNewURL {
+				t.Errorf("URLMap.GetFilesystemLocation() gotNewURL = %v, want %v", gotNewURL, tt.wantNewURL)
+			}
+			if gotFsPath != tt.wantFsPath {
+				t.Errorf("URLMap.GetFilesystemLocation() gotFsPath = %v, want %v", gotFsPath, tt.wantFsPath)
+			}
+		})
+	}
+}