Upgrade devise and devise-two-factor, remove devise-async

Devise (3.5.4 => 4.1.1) Changelog:
https://github.com/plataformatec/devise/blob/master/CHANGELOG.md

devise-two-factor (2.0.1 => 3.0.0) Changelog:
https://github.com/tinfoil/devise-two-factor/blob/master/CHANGELOG.md

These are reliant on each other, so they have to be upgraded together.

devise-async is no longer necessary as Devise 4.1 fixes a bug with the
ActiveJob integration.