Add option to map API tracking parameter to custom variable in log importer script.

Scheduled reports: do not show "Users" metric when value is zero 

Site Searches metric should include duplicate site searches requests

Refs #6436, #7112 fix performance regression, assume there are not duplicates in DB when doing segment query foraction since group by in sub-query is slow.

Refs #7151, add tests for new log importer options, fix small bug where userid not set in hit object, fix ignore group option value spliting.

When User ID is used, set the first party cookie UUID to this User ID

Updated translations from otrance

Refactored the SEO plugin to make it extensible and error-proof

fixes https://github.com/piwik/piwik/issues/7184

Monolog plugin

Some performance improvements for the all websites dashboard

a few performance tweaks as everything in that for loop is executed over...

Plugins can provide their own config files

"0" (the string) was casted to a boolean in the `if` which lead to it being `true`