Add support for custom activities (modmail-dev#3352)

(cherry picked from commit 26d73f5)

Author: sebkuip

cogs/utility.py

@@ -466,6 +466,7 @@ async def activity(self, ctx, activity_type: str.lower, *, message: str = ""):
             - `listening`
             - `watching`
             - `competing`
+            - `custom`
 
         When activity type is set to `listening`,
         it must be followed by a "to": "listening to..."
@@ -477,6 +478,9 @@ async def activity(self, ctx, activity_type: str.lower, *, message: str = ""):
         the linked twitch page:
         - `{prefix}config set twitch_url https://www.twitch.tv/somechannel/`
 
+        When activity type is set to `custom`, you can set
+        any custom text as the activity message.
+
         To remove the current activity status:
         - `{prefix}activity clear`
         """
@@ -576,7 +580,9 @@ async def set_presence(self, *, status=None, activity_type=None, activity_messag
         elif activity_type == ActivityType.streaming:
             url = self.bot.config["twitch_url"]
 
-        if activity_type is not None:
+        if activity_type == ActivityType.custom:
+            activity = discord.CustomActivity(name=activity_message)
+        elif activity_type is not None:
             activity = discord.Activity(type=activity_type, name=activity_message, url=url)
         else:
             activity = None