(timer): UI improvements.

Add `pomodoro_channel` guild setting.
Add customisable per-timer text channel.
Improve `reaction_message` flow.
Change algorithm for updating vc name.
Add `stage` and `pattern` vc name substitutions.

bot/modules/study/timers/settings.py

@@ -0,0 +1,47 @@
+import asyncio
+
+from settings import GuildSettings, GuildSetting
+import settings
+
+from . import Timer
+
+
+@GuildSettings.attach_setting
+class pomodoro_channel(settings.TextChannel, GuildSetting):
+    category = "Study Tracking"
+
+    attr_name = "pomodoro_channel"
+    _data_column = "pomodoro_channel"
+
+    display_name = "pomodoro_channel"
+    desc = "Channel to send pomodoro timer status updates and alerts."
+
+    _default = None
+
+    long_desc = (
+        "Channel to send pomodoro status updates to.\n"
+        "Members studying in rooms with an attached timer will need to be able to see "
+        "this channel to get notifications and react to the status messages."
+    )
+    _accepts = "Any text channel I can write to, or `None` to unset."
+
+    @property
+    def success_response(self):
+        timers = Timer.fetch_guild_timers(self.id)
+        if self.value:
+            for timer in timers:
+                if timer.reaction_message and timer.reaction_message.channel != self.value:
+                    timer.reaction_message = None
+                    asyncio.create_task(timer.update_last_status())
+            return f"The pomodoro alerts and updates will now be sent to {self.value.mention}"
+        else:
+            deleted = 0
+            for timer in timers:
+                if not timer.text_channel:
+                    deleted += 1
+                    asyncio.create_task(timer.destroy())
+
+            msg = "The pomodoro alert channel has been unset."
+            if deleted:
+                msg += f" `{deleted}` timers were subsequently deactivated."
+            return msg