When Piwik config file becomes un-readable, make it clear that the file still exists but is not readable (#10331)

* * In messages, make it more clear whether config file exists and/or is (not) readable
* When the config file exists but is not readable, provide users the choice between making the file readable, or starting a fresh install
* As long as Piwik is not installed, piwik.php should not return 500 when config file is not readable
* Fixes #10283
* UI tests: updated message when config file is not found

Allow plugins to have environment DI config files. Move all logging DI to Monolog plugin and only use logging in tracker if [Tracker] debug = 1.

Added environment validation system test (mostly failing) that tests Piwik's behavior when INI files are gone or corrupt from each Piwik endpoint (tracker/reporting UI/console). Hacked test code to make it possible and for some tests to pass.

This file is not in a class because it needs to be compatible with PHP 4.

#6085 Removed $GLOBALS['PIWIK_TRACKER_DEBUG_FORCE_SCHEDULED_TASKS'] and replaced it with DEBUG_FORCE_SCHEDULED_TASKS

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source

added special DeviceDetectorCache class used for caching DeviceDetector. This cache combines file and static caching to speed up detections as much as possible

added possibility to get/create a tracker by using "Tracker = AnalyticsTracker.getTracker" instead of "Piwik.getTracker". Variable will be set only if no such variable is defined yet

UserAgentParserEnhanced is now required and used from it's own composer package (piwik/device-detector); additionally Spyc library is required as composer package and removed from piwik source

refs #4780 added possibility to enable tracker debug via config, use logger for printing debug messages which allows users to log to screen, file or db

refs #4439 disable eAccelerator if enabled (just disabling optimizing did not work) and display a message in the admin UI. Also disabled eAccelerator during tracking. We might need to move the method to detect eAccelerator to another place, not sure if this is the best one