Match intent to ship threads that don't include a colon in the subject

[maxh]

Relevant line of code: https://github.com/GoogleChrome/blink-intent-tracker/blob/master/app-engine/handlers.py#L32

Example of thread that was missed: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Tn3pfJZDcGg