Add system_content for thread message types, fix other system_content issues

discord/message.py

@@ -994,21 +994,27 @@ class Message(Hashable):
         if self.type is MessageType.default:
             return self.content
 
-        if self.type is MessageType.pins_add:
-            return f'{self.author.name} pinned a message to this channel.'
-
         if self.type is MessageType.recipient_add:
-            return f'{self.author.name} added {self.mentions[0].name} to the group.'
+            if self.channel.type is ChannelType.group:
+                return f'{self.author.name} added {self.mentions[0].name} to the group.'
+            else:
+                return f'{self.author.name} added {self.mentions[0].name} to the thread.'
 
         if self.type is MessageType.recipient_remove:
-            return f'{self.author.name} removed {self.mentions[0].name} from the group.'
+            if self.channel.type is ChannelType.group:
+                return f'{self.author.name} removed {self.mentions[0].name} from the group.'
+            else:
+                return f'{self.author.name} removed {self.mentions[0].name} from the thread.'
 
         if self.type is MessageType.channel_name_change:
-            return f'{self.author.name} changed the channel name: {self.content}'
+            return f'{self.author.name} changed the channel name: **{self.content}**'
 
         if self.type is MessageType.channel_icon_change:
             return f'{self.author.name} changed the channel icon.'
 
+        if self.type is MessageType.pins_add:
+            return f'{self.author.name} pinned a message to this channel.'
+
         if self.type is MessageType.new_member:
             formats = [
                 "{0} joined the party.",
@@ -1030,16 +1036,28 @@ class Message(Hashable):
             return formats[created_at_ms % len(formats)].format(self.author.name)
 
         if self.type is MessageType.premium_guild_subscription:
-            return f'{self.author.name} just boosted the server!'
+            if not self.content:
+                return f'{self.author.name} just boosted the server!'
+            else:
+                return f'{self.author.name} just boosted the server **{self.content}** times!'
 
         if self.type is MessageType.premium_guild_tier_1:
-            return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 1!**'
+            if not self.content:
+                return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 1!**'
+            else:
+                return f'{self.author.name} just boosted the server **{self.content}** times! {self.guild} has achieved **Level 1!**'
 
         if self.type is MessageType.premium_guild_tier_2:
-            return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 2!**'
+            if not self.content:
+                return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 2!**'
+            else:
+                return f'{self.author.name} just boosted the server **{self.content}** times! {self.guild} has achieved **Level 2!**'
 
         if self.type is MessageType.premium_guild_tier_3:
-            return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 3!**'
+            if not self.content:
+                return f'{self.author.name} just boosted the server! {self.guild} has achieved **Level 3!**'
+            else:
+                return f'{self.author.name} just boosted the server **{self.content}** times! {self.guild} has achieved **Level 3!**'
 
         if self.type is MessageType.channel_follow_add:
             return f'{self.author.name} has added {self.content} to this channel'
@@ -1059,9 +1077,18 @@ class Message(Hashable):
         if self.type is MessageType.guild_discovery_grace_period_final_warning:
             return 'This server has failed Discovery activity requirements for 3 weeks in a row. If this server fails for 1 more week, it will be removed from Discovery.'
 
+        if self.type is MessageType.thread_created:
+            return f'{self.author.name} started a thread: **{self.content}**. See all **threads**.'
+
         if self.type is MessageType.reply:
             return self.content
 
+        if self.type is MessageType.thread_starter_message:
+            if self.reference is None or self.reference.resolved is None:
+                return 'Sorry, we couldn\'t load the first message in this thread'
+
+            return self.reference.resolved.content  # type: ignore
+
         if self.type is MessageType.guild_invite_reminder:
             return 'Wondering who to invite?\nStart by inviting anyone who can help you build the server!'