Fix Transitions UI test (don't use id attribute to check if row has subtable, use subDataTable class).

Refs #5355, use existing plus/minus icons in normal subtable reports and don't use new CSS for actions reports which use images.

Fixes #6079, move skip unique visitors calculation logic to small method in ArchiveProcessor\Rules and fix build.

Allow LogAggregator to work with multiple site IDs, compute unique visitors for archives of multiple sites and add option to control whether unique visitors for multiple sites are processed.

Fixes #6178, select columns of a table correctly in handleColumnHighlighting function in dataTable.js.

Fixes #5355, add plus/minus icons to left of rows w/ subtables that change when tables expand/contract. Also add tooltip to rows w/ subtables.

Fixes #6034, replace down arrow in segment selector top control with icon used in segment editor. Also fixed issue where selector size change would not move icon to the right.

Testing if /proc is a resource with @fopen fails on systems with open_basedir set.
 by using stat we not only test the existance of /proc but also confirm it's a 'proc' filesystem

User ID implementation 

new "Users" metric in graph picker https://github.com/piwik/piwik/issues/6162, JS code showing //host notation https://github.com/piwik/piwik/pull/344  adding optout iframe screenshot

fixing few more tests: hide nb_users to make tests pass because for some reason the column does appear by default but in the backwardsCompatibilityTest it does not appear there. Also had to manually activate and install plugins in the test case for some reason. Message was:
```
Exception: Failed to update database (errors or warnings found): Array
(
    [warnings] => Array
        (
        )

    [errors] => Array
        (
            [0] => /home/matt/dev/piwik-master/plugins/CustomAlerts/Updates/0.0.2.php:
Error trying to execute the query 'ALTER TABLE `alert` CHANGE `enable_mail` `email_me` BOOLEAN NOT NULL'.
The error was: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'piwik_tests.alert' doesn't exist
        )

    [coreError] =>
    [deactivatedPlugins] => Array
        (
            [0] => CustomAlerts
        )

)

```

and restore Transitions tests which I deleted by mistake earlier

refs #6162 Extending config setting enable_processing_unique_visitors_* to also affect the nb_uniq_users metric processing over periods, + only show Users metrics when it's greater than zero, + fix more tests and renamed test suite to UserIdAndVisitorIdTest

refs #6162 when aggregating periods, do not sum 'Unique Users' metric as this is not a valid operation.

Refs #6109 New core metric: nb_users as the count of distinct user ids. Means "Active Users". Displayed in Visitors Overview, in metrics picker, in API response, in Metadata

Conflicts:
	core/Version.php
	misc/package/build.sh

Refs #3490 Display User ID in the Visitor Profile, in the Visitor log and Last visits widget link/tooltip opening the Visitor profile