(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/__init__.py

@@ -1,2 +1,3 @@
 from .Timer import Timer
 from . import commands
+from . import settings