Adding support for localisation

Adds most of the code needed to support multiple translations using flask-bablex. Currently this is disabled behind a feature flag in config.

Translations for large chunks of the UI exist in German and Italian.

Still needs most of the strings in the application to be marked as translatable and to work out the process of accepting translations.

I think we should merge this now and leave the feature flag defaulted to false until we sort out the two points above.

Updates #5