Implement app lifecycle

[shangzhel]

Closes #54.

Interface changes:

• newCard is no longer a free function, and has become a callback of IAppContext. Calling it causes a blank card to be shown in the card detail pane. The callback does not return anything (() => void) because all React state changes are asynchronous.
• Calling update or commit on an ICard will throw unless it is the one obtained from the detail prop of Home.
• Calling commit on a detail ICard that is not new and has not been modified will resolve but not actually send a PATCH request.
  • This should be of no concern, because nothing should happen when nothing about a card had changed.

Also included:

• Server error handler
  • HttpStatusError is handled and the status code and message sent in response.
  • Anything else is masked with an HTTP 500 response with no detail to prevent implementation leakage.

Note:

• Although there are code files in the controllers subdirectory, all that can be found in them are interface definitions for the controllers.
  • Concrete implementations are found in web/src/App.tsx instead, because they are coupled with user state, where the card information is actually represented, and network activity.
  • If we want to refactor it, we can, but I doubt the effort will be worth especially for this sprint.

[chomosuke]

some random opinions about where things should be but other than that it looks good.

[shangzhel]

Merging it now because it's blocking literally everyone else.