Rethinking Shared Data

[bknoles]

Continuing the conversation from #108

A quick summary:

• The combination of "module level" shared data storage, and "controller level" setters for shared data has introduced tricky bugs including shared data leaking across threads and across requests.
• While we've patched these issues, it'd be great to refactor away from global shared state.
• Controller class variables have been suggested in the past.
• I'm wary of using controller class variables, since in production they are also a form of global shared state.
• @PedroAugustoRamalhoDuarte has put together #111 , which refactors inertia shared state into instance variables. These are set via before_action callbacks, called by inertia_share.