Fix translation of WTForms strings (simplified again)

This resolves the https://github.com/gae-init/gae-init-babel/issues/6 "Strings that are in WTForms are not translated" issue.

Note: this supersedes the previous https://github.com/gae-init/gae-init-babel/pull/12 (including feedback provided there) and https://github.com/gae-init/gae-init-babel/pull/10 pull requests.