Add full name field, make back/forward work better

This commit adds the "Full name" field, in a way that's compatible with our
medic-webapp user management. We also now use `express-session` directly to
remember information from previous screens, in order to behave better when users
use the back/forward wizard-style navigation.

This has been done for a couple days; I'm just now getting around to merging it.
There are some visual layer CSS fixes still left to merge, and then we should be
pretty much done with https://github.com/medic/medic-webapp/issues/724.