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/coderd.go

@@ -585,6 +585,8 @@ func New(options *Options) *API {
 		AppearanceFetcher: &api.AppearanceFetcher,
 		BuildInfo:         buildInfo,
 		Entitlements:      options.Entitlements,
+		Telemetry:         options.Telemetry,
+		Logger:            options.Logger.Named("site"),
 	})
 	api.SiteHandler.Experiments.Store(&experiments)