PLF-5968: Normal user can change branding settings

Fix description:
- Only allow administrator saving branding settings
- Use ConversationState to check if user is admin or not

Conflicts:
	extension/portlets/branding/src/main/java/org/exoplatform/platform/portlet/juzu/branding/BrandingController.java
	extension/portlets/branding/src/main/resources/locale/portlet/branding/branding_en.properties