Cancel button during measurement

[david-peeler]

Summary: Add a "cancel" button during measurements.

Details: Allow users to cancel the measurement process in case they change their mind.

[jasonccox]

I don't know if we actually want to support this. We need to be careful not to allow users to cherry-pick the data they send to the FCC. If the download test finishes and for some reason they don't like the measured speed, they might cancel. I suppose we could still record a partial measurement whenever one is canceled.