refactor: cache own user ID in IDiscordCache

- remove Cluster.User
- remove Cluster.Application (it was only being used as an alternative to Cluster.User for some reason)
- move Bot.PermissionsIn to DiscordCacheExtensions

Myriad/Cache/MemoryDiscordCache.cs

@@ -16,6 +16,7 @@ namespace Myriad.Cache
         private readonly ConcurrentDictionary<ulong, Role> _roles = new();
         private readonly ConcurrentDictionary<ulong, User> _users = new();
         private readonly ConcurrentDictionary<ulong, GuildMemberPartial> _guildMembers = new();
+        private ulong? _ownUserId { get; set; }
 
         public ValueTask SaveGuild(Guild guild)
         {
@@ -46,6 +47,15 @@ namespace Myriad.Cache
             }
         }
 
+        public ValueTask SaveOwnUser(ulong userId)
+        {
+            // this (hopefully) never changes at runtime, so we skip out on re-assigning it
+            if (_ownUserId == null)
+                _ownUserId = userId;
+
+            return default;
+        }
+
         public ValueTask SaveUser(User user)
         {
             _users[user.Id] = user;
@@ -125,6 +135,8 @@ namespace Myriad.Cache
             return default;
         }
 
+        public Task<ulong> GetOwnUser() => Task.FromResult(_ownUserId!.Value);
+
         public ValueTask RemoveRole(ulong guildId, ulong roleId)
         {
             _roles.TryRemove(roleId, out _);