rename secret to hidden

[QA'd by Hannah] 

I've done a major find-and-replace throughout the codebase. Would be good to pull this down locally and check that I haven't been over- or under-zealous. 

It's critical that this migration run as soon as this gets deployed. Test deploy on clone first:

```
git push loomio-clone rename-secret-to-hidden:master && heroku run rake db:migrate -a loomio-clone
```