Add member avatar edit command

This also refactors a large portion of the DI toolchain, since
I discovered that you shouldn't be reusing IDbConnection objects.

Instead, most services and stores are now declared transient, and
the webhook cache has been moved to a database-independent storage
singleton by itself.

PluralKit.Bot/Utils.cs

@@ -1,7 +1,7 @@
 using System;
-using System.Collections.Generic;
 using System.Data;
 using System.Linq;
+using System.Net.Http;
 using System.Threading.Tasks;
 using Dapper;
 using Discord;
@@ -9,7 +9,7 @@ using Discord.Commands;
 using Discord.Commands.Builders;
 using Discord.WebSocket;
 using Microsoft.Extensions.DependencyInjection;
-using NodaTime;
+using Image = SixLabors.ImageSharp.Image;
 
 namespace PluralKit.Bot
 {
@@ -17,6 +17,48 @@ namespace PluralKit.Bot
         public static string NameAndMention(this IUser user) {
             return $"{user.Username}#{user.Discriminator} ({user.Mention})";
         }
+
+        public static async Task VerifyAvatarOrThrow(string url)
+        {
+            // List of MIME types we consider acceptable
+            var acceptableMimeTypes = new[]
+            {
+                "image/jpeg",
+                "image/gif",
+                "image/png"
+                // TODO: add image/webp once ImageSharp supports this
+            };
+
+            using (var client = new HttpClient())
+            {
+                Uri uri;
+                try
+                {
+                    uri = new Uri(url);
+                    if (!uri.IsAbsoluteUri) throw Errors.InvalidUrl(url);
+                }
+                catch (UriFormatException)
+                {
+                    throw Errors.InvalidUrl(url);
+                }
+                
+                var response = await client.GetAsync(uri);
+                if (!response.IsSuccessStatusCode) // Check status code
+                    throw Errors.AvatarServerError(response.StatusCode);
+                if (response.Content.Headers.ContentLength == null) // Check presence of content length
+                    throw Errors.AvatarNotAnImage(null);
+                if (response.Content.Headers.ContentLength > Limits.AvatarFileSizeLimit) // Check content length
+                    throw Errors.AvatarFileSizeLimit(response.Content.Headers.ContentLength.Value);
+                if (!acceptableMimeTypes.Contains(response.Content.Headers.ContentType.MediaType)) // Check MIME type
+                    throw Errors.AvatarNotAnImage(response.Content.Headers.ContentType.MediaType);
+
+                // Parse the image header in a worker
+                var stream = await response.Content.ReadAsStreamAsync();
+                var image = await Task.Run(() => Image.Identify(stream));
+                if (image.Width > Limits.AvatarDimensionLimit || image.Height > Limits.AvatarDimensionLimit) // Check image size 
+                    throw Errors.AvatarDimensionsTooLarge(image.Width, image.Height);
+            }
+        }
     }
 
     class UlongEncodeAsLongHandler : SqlMapper.TypeHandler<ulong>