* add Email field to User entity (+codegen)
* switch to Echo contrib sessions backed by Gorilla sessions
* use SessionCookieSecret value from the config
* stage mod,sum changes
* add clearer signup/signin redirect logic
* render error pages on 500/404s and only fall back to returning raw
errors when the error is some unexpected kind
* add username/email "exists" funcs+tests - handle "not found" and "not
unique" errors, return errors otherwise
