In 62acfde: refs #5212 improved left menu stylesheet which works better with Morpheus now, turned it into a plugin and made it configurable. not finished yet

Faced some challenges here so far:

• As each user can enable/disable the LeftMenu the generated CSS is different for each user. If two or more users are using the same Piwik instance at the same time this results basically in regenerating the CSS all the time. This is a general issue and most likely needs a separate ticket. We could for instance persist different CSS files for each cacheBuster value: asset_....css$CB.css . Meaning we serve the correct CSS file to the correct users and do not have to recreate them all the time.
• A SystemSetting is currently only read AND writable by super users. Meaning I cannot access the default value whether by default the left menu is enabled or not if a user is not the super user. We need either a new type of SystemSetting which allows to READ for everyone but write only for SuperUsers or we modify the existing one. Not sure yet

In 82da315: refs #5212 added possibility to make a system setting readable for non super users

Found a solution for SystemSetting which is backwards compatible and prevents us from creating lots of classes as some SystemSettings should be maybe readable by anonymous users whereas some should be readable only for users having some view access etc. This would be also a proper solution for this hack: https://github.com/piwik/piwik/blob/master/core/Plugin/Settings.php#L330 ( #4924 )

To be defined whether we remove this hack

In bb4d352: refs #5212 do not display the default value in case we handle radio fields

Ideally there would be a setting to enable it only for large displays. Eg on tablets or mobile phones it does not make sense to have it on the left side. On the other side on large displays it makes sense. We could detect the screen size and enable the left menu client side. Would most likely also fix the problem that we do not have to regenerate the CSS all the time and we would get rid of some CSS !important definitions.

In 15f3cad: refs #5212 this should enable leftMenu plugin by default but it will be disabled. Need to test it

In 5bbf3b7: refs #5212 added some tests

In 7d1b664: refs #5212 should fix core tests

In 8fb7b4c: refs #5212 always add leftMenu css but apply it only if there is a class .leftMenuPlugin. Added an event to add classes to the body which I found quite useful already for a while

In 4e950e8: refs #5212 a few ui tweaks

In 859a682: refs #5212 css => less

In 0134372: refs #5212 translations

In 9a960ae: refs #5212 another improvement for combo of zenMode + leftMenu

In 8208b89: Refs #5212 add missing reference, this maybe was a merge-fail