Make it possible to switch to/from CDN without restart

[php-coder]

At this moment, we have app.use-cdn parameter (#1059) to enable/disable CDN usage. In order to be able to mitigate possible slowdowns or outages of external resources quicker (without restart or redeploy), let's make it configurable via Togglz.

Also, with the feature flag we can switch it on/off to measure performance of different strategies. Perhaps, we should serve everything from our own domains: https://csswizardry.com/2019/05/self-host-your-static-assets/

[php-coder]

Also, with the feature flag we can switch it on/off to measure performance of different strategies.

If we still want to use CDNs, we should play with a number of external domains. It seems that we have too many external domains and it might be beneficial to reduce this number.

[php-coder]

Perhaps, we should serve everything from our own domains: https://csswizardry.com/2019/05/self-host-your-static-assets/

In this case, we don't need to implement SRI (#244)