Add Head-to-Head kill stats

Add basic head-to-head kill analysis and the corresponding matrix display in the UI

migrations/2024-11-04-220702_head-to-head/up.sql

@@ -0,0 +1,8 @@
+-- Your SQL goes here
+CREATE TABLE IF NOT EXISTS demo_head_to_head (
+  demo_id TEXT NOT NULL,
+  player TEXT NOT NULL,
+  enemy TEXT NOT NULL,
+  kills int2 NOT NULL,
+  PRIMARY KEY (demo_id, player, enemy)
+);