Add [subject,user]_reducer_keys config field to reducer config UI

[lcjohnso]

Currently, the subject_reducer_keys configuration field on user reducers, and the user_reducer_keys configuration field on subject reducers are configurable only on the Rails console. Given there are no (known?) security issues with configuration a list of strings (similar to the reducer's extractor_keys filter config), it would be helpful to add this config field to the reducer config UI.