make DefaultContext private

[sio4]

DefaultContext implements buffalo.Context and this is the core context for buffalo's application flow, and it should be configured correctly to work properly with its functions. However, DefaultContext is currently public, so it can be used without careful configuration by initializing it directly. It could be better to make it private and just provide a generator function. (currently, App.newContext() is the generator)

TODO

• make DefaultContext private and only allow it to be generated by App.newContext() or other internal code.

Context

• #2241
• #2246