Return HTTP 204 instead of GIF for JavaScript tracking.

Refactor out HtmlMessageException 

[automatic translation update] Updated 179 strings in 7 languages (fr, nl, cs, el, da, de, it)

fixes #6663 New command `./console core:plugin list` will list the enabled plugins, one per line, making it easy to compare the list of plugins used in two piwik installs

 Do not store Accept-language full string, only store one detected language

added a new url parameter send_image=0 to disable sending an
image and instead response with a 204 code

Revert "3581 IPv6 support"

Add example archiver and CoreConsole task to generate Archiver into plugin.

Refs #6101, fix sumRowArray error caused by Provider change (GroupBy must be called as ->filter not ->queueFilter since it changes DataTable structure).

Easier configuration to run tests

#6635 rename phpunit.xml to phpunit.xml.dist. did not know it is supported out of the box by phpunit

refs #6635 if user has not configured test section but Piwik is installed then we can configure most parts for the user