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/Limits.cs

@@ -5,5 +5,8 @@ namespace PluralKit.Bot {
         public static readonly int MaxDescriptionLength = 1000;
         public static readonly int MaxMemberNameLength = 50;
         public static readonly int MaxPronounsLength = 100;
+
+        public static readonly long AvatarFileSizeLimit = 1024 * 1024;
+        public static readonly int AvatarDimensionLimit = 1000;
     }
 }