Rename Authentication template files

Follow-up to [#53098][]

The corresponding commit in `rails/rails` nested the template files
(including the view templates) under an `app/` directory.

This commit makes a corresponding change, nesting the `views/` templates
under `app/views/`.

[#53098]: rails/rails#53098 (comment)

Co-authored-by: Jerome Dalbert <jerome.dalbert@gmail.com>

Author: seanpdoyle