Webperf -Add HTTP "Cache-Control: immutable" headers for images files

Added HTTP "Cache-Control: immutable" headers for PNG, JPG and SVG files to use the browser's cache to the maximum.

• For Firefox, thhe HTTP "Cache-Control: immutable" header will be honored only if the content is served in HTTPS.
• Chrome does not actually support immutable at the moment, but has its own new heuristics

Documentation

• https://www.nicolas-hoffmann.net/source/1710-Deployer-cache-control-immutable-avec-du-cache-busting.html

<IfModule mod_headers.c>
    <FilesMatch "\.(png|svg|jpg|jpeg)$">
        Header set Cache-Control "public, max-age=31536000, immutable"
    </FilesMatch>
</IfModule>

Todo list

• add the timestamp on the software logo
• add the timestamp on the users logo
• add timestamp on screenshots
• refine the apache rule to handle only images from the following directories:
  • /webroot/img/logos
  • /webroot/img/files
• apply changes in prod (code + Apache configuration)

---

• same as #796 (closed)
• see also: !373 (merged)