Implement Steam Guard code generation

[bmwalters]

Refer to https://github.com/mattrubin/OneTimePassword/pull/225

Overview

• Adds a new token type option: "Steam Guard".
• Selecting the "Steam Guard" token type hides the algorithm and digit count controls, since there is only one set of valid parameters for generating Steam Guard tokens.
• Users are expected to figure out how to obtain their Steam Guard secret on their own.