Feature/ Frontend TOTP activation shortcode

[JurriaanK]

This implements a [two-factor-set-totp] shortcode which can be used for front-end TOTP 2fa activation.

I looked into making all provider settings available frontend, but that would require extensive modifications to the whole plugin. So I took a rather straightforward approach. I created specific functions for generating the shortcode output and saving the input because outputting to frontend and saving options from a frontend request demands a different approach than backend, but I tried to reuse as much of the backend functions as possible. I think this brings a much wanted feature in a relatively clean approach.

Notes:

• Currently only first time activation is possible. Reset of the key has to be done via the user profile. Allowing users to reset via frontend requires a global setting on admin level in my opinion, but two-factor currently doesn't have a global settings page yet.
• The shortcode doesn't use wp-admin urls, which is handy for sites where wp-admin is blocked for non-admin users.
• After using the frontend settings the TOTP provider is enabled and set as primary.

[jeffpaul]

Per yesterday's bug scrub, general sentiment was in favor of this but going to punt to future release so we can focus on the U2F deprecation in 0.8.0.

[iandunn]

Related #261