SiteAccesses outside Content's root tree are still displayed in Page tab and selecting them results in previewing their (SiteAccesse's) root Location, instead requested content.

This behavior is very misleading for Editors in projects with multiple SiteAccesses with different tree roots as many of displayed SiteAccesses can't be used for LandingPage preview/edit.

Steps to reproduce
1. Clean installation of eZ Platform 2.5.6
2. Log in to the Backoffice as admin. In Content tree root (Home) create and publish Folder Folder A.
3. In Content tree root (Home) create and publish Folder Folder B.
4. In Folder A create Landing Page Test Page and publish it.
5. Set up your project as follows (ezplatform.yml):

siteaccess:
        list: [site_a, site_b, admin]
        groups:
            site_group: [site_a, site_b]
            admin_group: [admin]
        default_siteaccess: site_a
        match:
            URIElement: 1

    system:
        admin_group:
            languages: [eng-GB]
            page_builder:
                siteaccess_list: [site_a, site_b, site]
            content_tree_module:
                contextual_tree_root_location_ids:
                    - 2 # Home (Content structure)
                    - 5 # Users
                    - 43 # Media
                    - 48 # Setup
                    - 55 # Forms
            subtree_paths:
                content: /1/2/
                media: /1/43/
        site_a:
            languages: [eng-GB]
            content:
                tree_root:
                    location_id: 56
                    excluded_uri_prefixes: [ /media, /images ]
        site_b:
            languages: [eng-GB]
            content:
                tree_root:
                    location_id: 57
                    excluded_uri_prefixes: [ /media, /images ]

6. In the Backoffice go to Test Page.
7. Click on the "Page" tab.

Result
There will be available 3 SiteAccesses: site, site_a, site_b. Clicking on site_b will result in previewing Folder B instead of Test Page without any warning, exception or alert.

Expected Result
There will be available 2 SiteAccesses: site, site_a.
site_b would be not on the list, as Test Page is outside of site_b content root tree.