Revert "Install ActiveAdmin"

This reverts commit 8f92b919eb6336ecf6aac57f23d3358bba8d2c96.
Use conventional namespaces for concerns

RailsAdmin requires it
Add options to User#image_url
Add profiling to rspec
Add recommended testing gems to gemfile
Add rails_admin with bootstrap 2 support

Removes bootstrap 3 from project
Update gems
Uncomment rails_admin initializer
Use fork of rails_admin to support Bootstrap 3

Fork statically includes bootstrap 2 so it does 
not conflict with bootstrap-sass gem
Override rails_admin nag to use user.image_url