Add user placeholders to "Default Values for New Submissions" fields

[benkeen]

This stemmed from #468. On reflection it's actually a real no-brainer of a feature: it will allow the Client Map filters to work seamlessly for users accessing a form with a Client Map filter confirmed.

Part of this ticket is to update the documentation for Client Map filters / Default Values for New Submission fields setting, and add a tutorial how to use the feature.

Also, this feature should work with the Extended Client Fields module, which will mean an update to that module.