EZP-28127: Create extendable left sidebar

[webhdx]

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

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,
    ],
])

[webhdx]

Notice: This requires #26 so 2 commits are not a part of this PR. I will rebase as soon as #26 gets merged. For the time being please do code review on 9314604 commit.

[alongosz]

9314604 looks good. Note that I'm not familiar with KnpMenu, so commenting "code-wise" only ;)

[bdunogier]

As for the PR about the top menu, having howtos for the feature would be very helpful. Could you add that, even if it is just a draft in the PR's description for the time being ?