Added invite-only projects (Issue #75)

Attempts to address #75, though I went a bit off spec. Attention @calonso-conabio and @aescobarr, there are number of changes to `en.yml` that need translating. I'm hoping to deploy this in about a week, so please add your translations as pull requests against the inviteonly branch soon if you want to get them in before deployment.

Also, I'm experimenting with using pull requests as notifications that translations need to happen. Thoughts?