fixed /app_settings.json show to use appinfo.

The response should now include current settings and defaults.
added default app_settings file.

Default app_settings values. This file will get merged with the actual
`app_settings` property on the design doc.

If you add a new value to kanso.json settings_schema then you should update
this file also.  If you forget to update this then new defauls won't show up
in settings until it gets saved in the dashboard. This can be an issue if
you have a new feature that expects a setting default value.
fixed messages_invalid_custom and added app_settings_has_defaults tests.
refactored appinfo.js to use app_settings.js as defaults.

modified appinfo.translate to accept a context parameter, to do template
interpolation in translations using mustache formatting.
Merge branch 'develop' of github.com:medic/kujua-lite into develop