Add a variant which sets up 2FA with devise

[eoinkelly]

Current status

WIP. This branch can be used as inspiration for a devise MFA feature. The design doc included in this PR is particularly useful for understanding the feature.

Background

These changes were initially sketched in ackama/rails-template-demo-devise-2fa#11 and refined in some real client apps. Hat tip to @joshmcarthur who greatly improved my first pass at this. Fixes #61

TODO

• Be consistent in whether we describe this as 2FA or MFA
• Decide whether this variant should create a seed user for easy testing of sign-in and sign-out (decided against doing this)
• Fix all TODO items in the code
• Generate tests for all code added by this variant

[eoinkelly]

The basics are in this now but we need to add:

• Screens for managing OTP for your user
• A screen for backup codes (not sure how hard this is to do)

My goal here is that this should generate everything you need except CSS styling for the feature

[eoinkelly]

I now think this is big enough to be a whole separate template so I'm going to close this.

[eoinkelly]

This work has to be paused until devise-two-factor supports Rails 7. I'm trying to help make that happen on devise-two-factor/devise-two-factor#206

[eoinkelly]

See ackama/rails-template-demo-devise-2fa#1 for progress on this feature.

Edit: this is outdated, latest changes are in this PR now

[robotdana]

copy improvements from another project which doesn't need js and is instead an actual 2-step process with separate pages

[eoinkelly]

I have copied the files from ackama/rails-template-demo-devise-2fa#11 which incorporates changes made on client projects. This PR is still very WIP.