(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.

data/schema.sql

@@ -78,7 +78,8 @@ CREATE TABLE guild_config(
   returning_message TEXT,
   starting_funds INTEGER,
   persist_roles BOOLEAN,
-  daily_study_cap INTEGER
+  daily_study_cap INTEGER,
+  pomodoro_channel BIGINT
 );
 
 CREATE TABLE ignored_members(