Fix emby/user/public API leaking private data

This commit fixes the emby/user/public API that was returning more data
than necessary. Now only the following information are returned:
- the account name
- the primary image tag
- the field hasPassword
- the field hasConfiguredPassword, useful for the first wizard only
(see
https://github.com/jellyfin/jellyfin/issues/880#issuecomment-465370051)
- the primary image aspect ratio

A new DTO class, PrivateUserDTO has been created, and the route has been
modified in order to return that data object.

MediaBrowser.Controller/Library/IUserManager.cs

@@ -143,6 +143,14 @@ namespace MediaBrowser.Controller.Library
         /// <returns>UserDto.</returns>
         UserDto GetUserDto(User user, string remoteEndPoint = null);
 
+        /// <summary>
+        /// Gets the user public dto.
+        /// </summary>
+        /// <param name="user">Ther user.</param>\
+        /// <param name="remoteEndPoint">The remote end point.</param>
+        /// <returns>A public UserDto, aka a UserDto stripped of personal data.</returns>
+        PublicUserDto GetPublicUserDto(User user, string remoteEndPoint = null);
+
         /// <summary>
         /// Authenticates the user.
         /// </summary>