Change Max Channel per Category to 49

Addresses modmail-dev#3002
Will add a 1 channel buffer to the check to account for Discord API bugs preventing the category from actually reaching 50 channels.

core/thread.py

@@ -1205,7 +1205,7 @@ async def create(
 
         # Schedule thread setup for later
         cat = self.bot.main_category
-        if category is None and len(cat.channels) == 50:
+        if category is None and len(cat.channels) == 49:
             fallback_id = self.bot.config["fallback_category_id"]
             if fallback_id:
                 fallback = discord.utils.get(cat.guild.categories, id=int(fallback_id))