add entity header controls, content state mode

[anncatton]

Implements EDITING and CREATING content modes

• entity context will track content mode
• adds setMode to click event in Edit, Create, Cancel and Save buttons. No staged entity/api actions implemented
• adds separate form components for editing and creation
• fixes entity header display so it will render whether there is a currentId or not
• some enum and type fixes
• renames Entity type files This implementation doesn't use any of the Form structure provided in Semantic UI, just Inputs and Dropdowns. It might be worth investigating as a future enhancement if it helps introduce things like validation/error messaging.