mod_signup: Allow signups with email only

Added a configuration flag to set the username equal to the entered
email address on submit.

Also document the other configuration values that can be set.
mod_admin_config: Truncate large config values in the config table

This makes the table more manageable
translations: Chinese(zh) translations by Shuyu Wang aka andelf
mod_translation: Add zh to languages list, add Shuyu Wang to translators

Fixes #498
mod_admin_identity: Show inactive users in a lightgrey color

Thanks for the hint, Alexey Marunin :)