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.Model/Dto/PublicUserDto.cs

@@ -0,0 +1,48 @@
+using System;
+using MediaBrowser.Model.Configuration;
+using MediaBrowser.Model.Users;
+
+namespace MediaBrowser.Model.Dto
+{
+    /// <summary>
+    /// Class PublicUserDto. Its goal is to show only public information about a user
+    /// </summary>
+    public class PublicUserDto : IItemDto
+    {
+        /// <summary>
+        /// Gets or sets the name.
+        /// </summary>
+        /// <value>The name.</value>
+        public string Name { get; set; }
+
+        /// <summary>
+        /// Gets or sets the primary image tag.
+        /// </summary>
+        /// <value>The primary image tag.</value>
+        public string PrimaryImageTag { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this instance has password.
+        /// </summary>
+        /// <value><c>true</c> if this instance has password; otherwise, <c>false</c>.</value>
+        public bool HasPassword { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this instance has configured password.
+        /// </summary>
+        /// <value><c>true</c> if this instance has configured password; otherwise, <c>false</c>.</value>
+        public bool HasConfiguredPassword { get; set; }
+
+        /// <summary>
+        /// Gets or sets the primary image aspect ratio.
+        /// </summary>
+        /// <value>The primary image aspect ratio.</value>
+        public double? PrimaryImageAspectRatio { get; set; }
+
+        /// <inheritdoc />
+        public override string ToString()
+        {
+            return Name ?? base.ToString();
+        }
+    }
+}