Modify jqPlot to support retina displays.

fixes #4386 fix datepicker is not completely visible. I wanted to fix this in JS but unfortunately the beforeShow event of the datepicker is not working and I did not find any other workaround. So fixing it this way

refs #4393 reports added to the scheduled reports do not appear as checked. still looking for a better solution

fix Declaration of UIAssetCacheBusterMock::piwikVersionBasedCacheBuster() should be compatible with Piwik\AssetManager\UIAssetCacheBuster::piwikVersionBasedCacheBuster($pluginNames = false)

this way we will get at least the actual error message. Fix there are 2 constructors defined in some PHP versions (log() and __construct())

fixes #4389

[automatic translation update] Updated 443 strings in 6 languages (ca, de, el, fi, it, nl)

better comment, self served certicate tolerated here because it's only used to test server is working during tests

Reverting part of https://github.com/piwik/piwik/commit/641e9ce4aca81ddb3d66983aab80e477727a1d93

Fix regression in metrics picker: remove available column check. Use of 'columns' query parameter in API methods makes this check problematic.