feat: block running commands / proxying when system is being deleted

Large systems take way too long to delete, which causes any insert for that system to block a connection, which exhausts the connection pool

PluralKit.Core/Database/Functions/functions.sql

@@ -1,6 +1,7 @@
 create function message_context(account_id bigint, guild_id bigint, channel_id bigint)
     returns table (
         system_id int,
+        is_deleting bool,
         log_channel bigint,
         in_blacklist bool,
         in_log_blacklist bool,
@@ -27,6 +28,7 @@ as $$
         guild as (select * from servers where id = guild_id)
     select
         system.id as system_id,
+        system.is_deleting,
         guild.log_channel,
         (channel_id = any(guild.blacklist)) as in_blacklist,
         (channel_id = any(guild.log_blacklist)) as in_log_blacklist,