EZEE-2111: Landing Page field is visible while creating new Page with COTF

[alongosz]

Question	Answer
Tickets	https://jira.ez.no/browse/EZEE-2111
Requies	ezsystems/ezplatform-page-builder#98
Bug fix?	yes
New feature?	yes
BC breaks?	no
Tests pass?	yes
Doc needed?	no
License	GPL-2.0

This PR exposes SiteAccess-aware setting to inject custom Twig Form Theme into Admin UI Content edit forms.

Usage:

ezpublish:
  system:
     admin_group: # configuration per SiteAccess or SiteAccess group
         admin_ui_forms:
             content_edit_form_templates: 
                 - { template: 'my_template.html.twig', priority: 0 }

The higher the priority, the higher the template will be placed on the list.

Checklist:

• Create SiteAccess-aware settting
• Create default configuration for the new setting.
• Expose set Form Themes list in the admin_ui_config.contentEditFormTemplates Twig global variable.
• Introduce priorities to injected templates.
• Create unit test for mapping prioritized templates.
• Coding standards ($ composer fix-cs)
• Ready for Code Review

[alongosz]

Note: I'm gonna change config to allow prioritizing templates, as agreed discussing it internally

[alongosz]

@webhdx @mikadamczyk @Nattfarinn PR updated with support for prioritized templates and unit test checking that.