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.go

@@ -0,0 +1,30 @@
+package utils
+
+import "strings"
+
+// URLMap is a map of URL prefixes to filesystem locations
+type URLMap map[string]string
+
+// GetFilesystemLocation returns the adjusted URL and the filesystem location
+func (m URLMap) GetFilesystemLocation(url string) (newURL string, fsPath string) {
+	newURL = url
+	if m == nil {
+		return
+	}
+
+	root := m["/"]
+	for k, v := range m {
+		if k != "/" && strings.HasPrefix(url, k) {
+			newURL = strings.TrimPrefix(url, k)
+			fsPath = v
+			return
+		}
+	}
+
+	if root != "" {
+		fsPath = root
+		return
+	}
+
+	return
+}