EZP-28127: Create extendable left sidebar

[webhdx]

JIRA: https://jira.ez.no/browse/EZP-28127
Requires: #26

!! Reopening this one after it was merged too early !!

Description

This PR reimplements left sidebar (which is visible in Location View and Trash) using knplabs/knp-menu-bundle package. I also created extensibility point based on events.
Developers can take advantage of ConfigureMenuEvent::CONTENT_SIDEBAR_LEFT event to make customization to the sidebar i.e. reorder, add or remove items. Code snippets on how to customize sidebar are available at #26 as managing sidebar items uses the same approach.

All items are styled by default, more advanced buttons can use custom attributes as shown below:

$this->createMenuItem('sidebar_left__browse', [
    'extras' => ['icon' => 'browse'],
    'attributes' => [
        'class' => 'btn--udw-browse',
        'data-starting-location-id' => 1,
    ],
])

TODO

• merge EZP-28117: Redesign menu system in AdminUI to event based approach for better extensibility #26 first
• rebase after EZP-28117: Redesign menu system in AdminUI to event based approach for better extensibility #26 has been merged

[webhdx]

Please do code reviews on cd5eacd commit only as other commits are from #26. I will rebase once #26 gets merged.