Revert "CLC-4366 Remove some unused junk"

This reverts commit 342a6e9689a7a1ee1a1db3438ec69f3ae95816f8.
Revert "CLC-4366 Move rails_admin into a Rails engine with its own Gemfile"

This reverts commit 8519f5acf6ab3a005b8a04683d85286eedf8ed09.
CLC-4366 Revert - remove Gemfile.lock noise
Merge pull request #3073 from ctweney/clc-4366-revert

CLC-4366 Reverting rails_admin engineification