Issues 3169 (Inconsistent labeling on Request an invite page) and 3175 (Add help text on account creation page)

https://code.google.com/p/otwarchive/issues/detail?id=3169 and https://code.google.com/p/otwarchive/issues/detail?id=3175

Rebase of https://github.com/otwcode/otwarchive/pull/762 as part of an effort to fix abandoned PRs. Verified in browser and my tests are passing locally.