Allow to validate Apache/Nginx configuration for image style files access

Problem/Motivation

See #3384272-5: Allow to validate Apache/Nginx configuration requirement for aggregation folder before enabling aggregation. →

Proposed resolution

In addition to the `Flush` image style operation, introduce the `Validate configuration` button to check if a particular image style generation works according to server requirement expectations.

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet