Revert "Remove admin"

This reverts commit 26150376fe99ec4c872c52be269265894423a7d8.
Clean up admin
Add README note for backend client
Rename backend-client to backend_client
Add backend client to admin
Add app initializer and config
Fix proxy setter bug
Require explicit whitelisting of update params
Update README
Merge tag 'v0.5.1' into develop

Release 0.5.1
Require explicit whitelisting of update params
Add app/controllers/concerns and app/models/concerns folders
Add mutations gem
Raise bad request error if type is invalid or not found
Remove Sinatra::RespondTo
Remove process_request and use error handlers instead
Provide detailed error messages for a few failure cases
Use I18n and override default error messages
Add endpoint to return a batch by id
Add service objects to list paginated resources