Add ability to add submission questions and interest topics in multiple languages

[osdiab]

right now they are just strings; for now, I just made it so those strings contain both translations, but it's not ideal and as more languages come in it is unscalable.

Proposal

1. add migration to change fields in topics and submissions so that:
   a) rather than containing the message directly, they contain a message ID
   b) add a JSON field to them called translations that is a mapping from language to text.
2. For these dynamically tranlsated things, don't use react-intl; just make the corresponding views display a value by conditioning on state.language.currentLanguage.