Handle accidental submissions

[arky]

We need a better way to handle accidental submissions. It is a common problem on touch interfaces: a user might accidentally mark a road as flooded while using zooming, panning touch gestures.

Reported in downstream code: nickmcintyre#1

The current workaround is to tap again to mark the road as flooded. This could be documented in the usage dialog box next to 'Zoom in to your street and click/tap to report it as flooded (or cleared)'.

In the long term, it would be good to provide a change history. The user might choose to revert certain changes before submitting the reports.