add translation_strategy feature flag to app

'dump-known' is the current behavior
'simple' is new behavior that simply doesn't attmept to outsmart the built-in translations

we need to test the possible ramifications of this before exposing
add optional 'preview' flag to app settings

and give it its first member
fix details of 'simple' translation strategy

exclude user-defined translations from default
move preview settings into advanced and add a "Preview" label
Merge pull request #1101 from dimagi/translation-strategy

Translation strategy option