fix: ensure websocket close messages are truncated to 123 bytes (#779)

It's possible for websocket close messages to be too long, which cause
them to silently fail without a proper close message. See error below:

```
2022-03-31 17:08:34.862 [INFO]	(stdlib)	<close_notjs.go:72>	"2022/03/31 17:08:34 websocket: failed to marshal close frame: reason string max is 123 but got \"insert provisioner daemon:Cannot encode []database.ProvisionerType into oid 19098 - []database.ProvisionerType must implement Encoder or be converted to a string\" with length 161"
```

coderd/httpapi/httpapi_test.go

@@ -5,8 +5,10 @@ import (
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/coder/coder/coderd/httpapi"
@@ -142,3 +144,23 @@ func TestReadUsername(t *testing.T) {
 		})
 	}
 }
+
+func WebsocketCloseMsg(t *testing.T) {
+	t.Parallel()
+
+	t.Run("TruncateSingleByteCharacters", func(t *testing.T) {
+		t.Parallel()
+
+		msg := strings.Repeat("d", 255)
+		trunc := httpapi.WebsocketCloseSprintf(msg)
+		assert.LessOrEqual(t, len(trunc), 123)
+	})
+
+	t.Run("TruncateMultiByteCharacters", func(t *testing.T) {
+		t.Parallel()
+
+		msg := strings.Repeat("こんにちは", 10)
+		trunc := httpapi.WebsocketCloseSprintf(msg)
+		assert.LessOrEqual(t, len(trunc), 123)
+	})
+}