Improve caching, HTTP headers and URL handling (#3594)

* Fix relative URLs
* Improve login base URL and redirects
* Prevent duplicate customlocales requests
* Improve UI base URL handling
* Improve UI embedding
* Improve CSP header
* Add Cache-Control headers to all responses
* Improve CORS responses
* Improve authentication handler
* Add back media timestamp suffixes
* Fix default image handling
* Add default param to other image URLs

pkg/ffmpeg/stream_transcode.go

@@ -260,6 +260,7 @@ func (sm *StreamManager) getTranscodeStream(ctx *fsutil.LockContext, options Tra
 
 	mimeType := options.StreamType.MimeType
 	handler := func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Cache-Control", "no-store")
 		w.Header().Set("Content-Type", mimeType)
 		w.WriteHeader(http.StatusOK)