Refine the “Security and Authentication” page

[chasenlehara]

• Update AuthContext to have state for when there is an error or a request isPending.
• Also, use the new state above to improve the UI.
• Use “Place an order” in the Restaurant Details view because it’s the button goes to the Order page (where there’s a “Place My Order!” Button).
• Use “Sign out” (sentence case) to be consistent with our other buttons.
• Split the “Welcome back” text so it looks better.
• Other minor edits.