Restructure db schema

The "commit_links" table is now called "commit_edges".

There is now a "metrics" table that run measurements have a foreign key
to. This provides canonical metric metadata and will speed up the
initial graph page (before any real data arrives). For now, it will be
overwritten with each new run, but more nuanced config options may be
added later.

.sqlx/query-249aa7a29ae6f1746b9c56ff4fbe9038a48ee5f32116a163af60617e0394de9c.json

@@ -0,0 +1,20 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT name FROM metrics ORDER BY name ASC",
+  "describe": {
+    "columns": [
+      {
+        "name": "name",
+        "ordinal": 0,
+        "type_info": "Text"
+      }
+    ],
+    "parameters": {
+      "Right": 0
+    },
+    "nullable": [
+      false
+    ]
+  },
+  "hash": "249aa7a29ae6f1746b9c56ff4fbe9038a48ee5f32116a163af60617e0394de9c"
+}