Update local environments with WebAuthn plugin

[iandunn]

See #114

After #153, some local environments like https://github.com/WordPress/wordcamp.org will fatal because the plugin is missing. We should make sure they're up to date with all the latest two-factor and wporg-two-factor related changes.

Props @ryelle for noticing

[dd32]

I was thinking that we should probably also bail early from the loading function if the plugin isn't available.

Perhaps throwing a warning if the environment is production.