Add support for permissions on Pages & Posts

[tidyui]

Add built-in support for specifying permissions for accessing pages & posts through the front-end application. The basic policy authentication in ASP.NET will be used (i.e the same that is used by the manager interface).

[tidyui]

Suggested implementation. A new property will be added to PermissionItem so that internal permissions can be distinguished from application permissions. These are added in the startup:

App.Permissions["Application"].Add(new Piranha.Security.PermissionItem
{
    Name = "Subscriber",
    Title = "Subscriber"
});
App.Permissions["Application"].Add(new Piranha.Security.PermissionItem
{
    Name = "Moderator",
    Title = "Moderator"
});

Please note that the name "Application" can be any name you'd like to use for grouping in the role management. These permissions can then be access when building roles in the manager:

Permissions that should only be used in the manager should be added as internal:

App.Permissions["Manager"].Add(new Piranha.Security.PermissionItem
{
    Category = "My category",
    Name = "Moderator",
    Title = "Moderator",
    IsInternal = true
});

When editing a page or post, the permissions which are not set as internal will be available on the settings modal.