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/testdata/fixtures/000288_telemetry_items.up.sql

@@ -0,0 +1,4 @@
+INSERT INTO
+    telemetry_items (key, value)
+VALUES
+    ('example_key', 'example_value');