Add 'default' option to the number field

> Created by: Neal Stammers (neal@netgorilla.co.uk) on 2015/03/10 18:02:16 +0000
> Votes at time of UserVoice import: 10

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When adding a number field you can currently set the min and max, but it would be useful to also be able to set a default value.

Thanks, Neal.
