chore: track the first time html is served in telemetry (#16334)

Addresses https://github.com/coder/nexus/issues/175.

## Changes

- Adds the `telemetry_items` database table. It's a key value store for
telemetry events that don't fit any other database tables.
- Adds a telemetry report when HTML is served for the first time in
`site.go`.

coderd/database/migrations/000288_telemetry_items.up.sql

@@ -0,0 +1,6 @@
+CREATE TABLE telemetry_items (
+    key TEXT NOT NULL PRIMARY KEY,
+    value TEXT NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW()
+);