(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/data.py

@@ -4,6 +4,7 @@ from data import RowTable
 timers = RowTable(
     'timers',
     ('channelid', 'guildid',
+     'text_channelid',
      'focus_length', 'break_length',
      'inactivity_threshold',
      'last_started',