Add Icons application for serving website icons.

src/Icons/Controllers/IconController.cs

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+using System;
+using System.IO;
+using System.Net.Http;
+using System.Runtime.Serialization.Formatters.Binary;
+using System.Threading.Tasks;
+using Icons.Models;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Icons.Controllers
+{
+    [Route("[controller]")]
+    public class IconController : Controller
+    {
+        private readonly IHostingEnvironment _hostingEnvironment;
+
+        public IconController(IHostingEnvironment hostingEnvironment)
+        {
+            _hostingEnvironment = hostingEnvironment;
+        }
+
+        [HttpGet]
+        public async Task<FileResult> Get([FromQuery] string domain)
+        {
+            var uri = BuildUrl(domain);
+            var fileName = $"{_hostingEnvironment.ContentRootPath}/cache/{domain}.cache";
+
+            // Attempt to load the icon from the cache.
+            if (FileExists(fileName))
+            {
+                using (Stream stream = System.IO.File.Open(fileName, FileMode.Open))
+                {
+                    var binaryFormatter = new BinaryFormatter();
+                    var icon = (Icon)binaryFormatter.Deserialize(stream);
+
+                    if (icon.HasNotExpired())
+                    {
+                        return new FileContentResult(icon.Image, icon.Format);
+                    }
+                }
+            }
+
+            var httpClient = new HttpClient();
+            var response = await httpClient.GetAsync(uri);
+
+            if (!response.IsSuccessStatusCode)
+            {
+                throw new Exception("Cannot load the image");
+            }
+
+            // Serialize the icon.
+            using (Stream stream = System.IO.File.Open(fileName, FileMode.Create))
+            {
+                var icon = new Icon(
+                    await response.Content.ReadAsByteArrayAsync(),
+                    response.Content.Headers.ContentType.MediaType
+                );
+
+                var binaryFormatter = new BinaryFormatter();
+                binaryFormatter.Serialize(stream, icon);
+                return new FileContentResult(icon.Image, icon.Format);
+            }
+        }
+
+        private static bool FileExists(string fileName)
+        {
+            return System.IO.File.Exists(fileName);
+        }
+
+        private static string BuildUrl(string domain)
+        {
+            return $"https://icons.bitwarden.com/icon?url={domain}&size=16..24..200";
+        }
+    }
+}