Add default view rights to guest for viewing the homepage/login

A common usecase for a Public Website is the one where the entire wiki is not viewable by the guest user, except for the homepage, which generally contains some instructions or description of the wiki/project.

When trying to setup rights to achieve the above usecase, one is faced with various technical details related to the fact that the rendering of the homepage requires view on a lot of included/related pages like the:

• skin
• flamingo theme
• panels
• pages/macros used/included in the homepage's content

For the things included in the homepage's content, it's easier for the admin to follow, but the rest are quite hidden and perhaps we can provide some default rights for guest users to avoid complications to admins when they want to restrict guests from accessing the wiki's content.

There are other important pages inside the XWiki space that should not be affected by rights changes on the wiki, like XWIKI-11205 describes.