Milo: Limit text field to 256 characters
Milo: Limit text field to 256 characters

To fix the string of 500 errors in the pubsub handler.

BUG=

Review-Url: https://codereview.chromium.org/2993623002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go