Issue #452 - Add allowExternalSources flag

* Define a new attribute for Question kind
* Display the option in the Dashboard - per question
* Include the flag in the xml Survey definition
Issue #452 - Disable 'externalSources' option for non-text questions
Issue #452 - Only send 'true' allowExternalSources flag
Experimental Dashboard using react and react-router
Skeleton cljs/om dashboard
Basic routing using secretary and multimethods
devices-list rendering
Remove some dead code
Introduce sablono
more sablono
Initial work on Users tab
Add missing comma
Add ability to add new user
edit user
delete user
urls for add/edit/delete user
Reusable dialog component
Server communication for add/update/delete users
Updated dialog example usage
Grid WIP