Add history table and modify card_stats

sql/history.sql

@@ -0,0 +1,11 @@
+create table history
+(
+    id INTEGER
+        primary key autoincrement,
+    set_id INTEGER not null references card_set on delete cascade,
+    stat_id INTEGER not null references card_stats on delete cascade,
+    timestamp INTEGER not null,
+    answer INTEGER not null,
+    before INTEGER not null,
+    after INTEGER not null
+);