CE-1975: add UserImpersonator and common args for stories

Adds the ability to impersonate the currently logged-in user in storybook stories by mocking-out the UserProvider s.t. you can change the roles, user properties, etc of the current user.

• Adds UserImpersonator component and userImpersonatorArgs util args to storybook stories
• Modifies test/story AppHarness component to toggle using the UserImpersonator and to pass args through to it

NOTE: Recall that on stories such as the UserPage the user displayed is fetched from the API. The UserImpersonator ONLY impersonates the logged-in user (the one in the app bar in the upper right), not the user displayed in the profile page!

Closes CE-1975