Issue #379 - methods to only load option questions without options

* For charts, we don't need the complete list of questions, just the
option question ones. And for those we don't need the actual options,
just the names.
Issue #379 - prevent unnecessary loading of questions
Issue #379 - prevent showing preview with incomplete questions

* if we have just shown a chart for a survey we are now previewing,
there will be incomplete questions in the datastore. We want to 
wait until the new results arrive before showing the preview.
Merge branch 'develop' of https://github.com/akvo/akvo-flow into develop