3169 3175 Labeling on request invite, help on user name & password

https://code.google.com/p/otwarchive/issues/detail?id=3169
https://code.google.com/p/otwarchive/issues/detail?id=3175

3169 Inconsistent labeling on Request an invite page - Removed landmark class that was hiding a heading, renamed button for looking up an email address, added translation strings. Didn't change the duplicate email fields, but that's okay -- I think the back end needs a bit of tweaking for that, too, to get the ids and names right, so let's bump it to a new issue.

3175 add help text on account creation page - Made a note of how long passwords must be and of how long user names must be and which characters they can contain. This applies to both the account creation page and the change user name and change email pages.

Replaces https://github.com/otwcode/otwarchive/pull/1477 which replaced https://github.com/otwcode/otwarchive/pull/762