Fix bug in referrers index.twig. Check that '...Percent' variables are defined before checking if not empty.
Fix twig bug in visitor log.
Refs #4041, cleaned up ViewDataTable, related classes, and related templates:

- Remove uniqIdTable, chartDivId and related code as important element IDs are determined in datatable_manager.js now.
- Clarify _dataTable twig template + related css.
- Also added visiblity to Piwik_DataTable_Renderer_Php::renderException.
Initialisation of the PleineLune theme
Add Less composer dependency
Use less to compile CSS files. CSS files are now always merged, even in development platform
Use the AssetManager constant to avoid typo
Change the extension to .less for CSS files that will be compiled
Remove old vendor-prefix for border-radius
Regenerate the merged CSS every time in development
Removed _dataTableActions.twig after making it possible to do so.
Fix small regression in last commit.
Handle @import rule in less files
Remove useless breaklines
Improve admin theming
Add PleineLune for main navigation
Migration of 'Add a site' button icon
Merge branch 'master' of github.com:piwik/piwik into pleineLune__less-installation
Modified _dataTable.twig to use a Piwik_DataTable instance instead of a DataTable converted to an array, allowed HtmlTables to render expanded DataTables, and removed @CoreHome/_dataTableActions_recursive.twig.
Fix regression in last commit.