gems: Generalise gem transaction table.

Updates gem audit table for more generality.
Fixes issue with `Row._refresh()`.
Data migration v11 -> v12.
Remove `set_gems` (not needed due to gem module refactor).

data/migration/v11-v12/migration.sql

@@ -0,0 +1,27 @@
+-- Add gem support
+ALTER TABLE user_config ADD COLUMN gems INTEGER DEFAULT 0;
+
+-- LionGem audit log {{{
+CREATE TYPE GemTransactionType AS ENUM (
+  'ADMIN',
+  'GIFT',
+  'PURCHASE',
+  'AUTOMATIC'
+);
+
+CREATE TABLE gem_transactions(
+  transactionid SERIAL PRIMARY KEY,
+  transaction_type GemTransactionType NOT NULL,
+  actorid BIGINT NOT NULL,
+  from_account BIGINT,
+  to_account BIGINT,
+  amount INTEGER NOT NULL,
+  description TEXT NOT NULL,
+  note TEXT,
+  reference TEXT,
+  _timestamp TIMESTAMPTZ DEFAULT now()
+);
+CREATE INDEX gem_transactions_from ON gem_transactions (from_account);
+-- }}}
+
+INSERT INTO VersionHistory (version, author) VALUES (12, 'v11-v12 migration');