Closes #163

Time-based one-time password (TOTP) codes are only valid for a short period
of time. In addition they are meant to be single-use to make them more
resistant against phishing and eavesdropping (e.g. keyloggers). Prior to this
change uffd did not keep track of used codes and thus did not prevent code
reuse.

Restructure tests into views/models/commands subdirectories to mirror the new
source tree structure introduced with ac731bf4 (Restructure source tree).