[2.2] Clean up Web app guide

[cllns]

Some of these are simple fixes for mistakes, but others are my preferences for explaining what's happening. I know we don't want to overload the reader, but I think some info can be helpful. Feel free to push back, or only take some of thse.

I also updated the screenshot, but edited the DOM so hopefully we can use that same image for all our 2.2 releases, instead of having to make a new one with each patch release to keep it accurate.

I'll also go through the API one, but I want to get feedback on this one first.

[cllns]

Some other notes I took:

• Should we change the book titles for switching from the fixtures to the DB records? Right now it's just the , by AUTHOR that's different, which could be hard to pick up on, since the titles are there in both, and the same.

• What do you think about renaming the feature specs to be what feature they're implementing for the user/app rather than their implementation? This might be too tied to my personal workflow but I find having distinct names helps when fuzzy searching files. Right now fuzzy-finding on books/index shows 6 files.
  Here's what I propose:

  spec/features/books/create_spec.rb           -> spec/features/add_book_spec.rb
  spec/features/books/index_spec.rb            -> spec/features/list_books_spec.rb
  spec/features/books/index/pagination_spec.rb -> spec/features/paginate_books_spec.rb
  spec/features/books/show_spec.rb             -> spec/features/show_book_spec.rb
  

• Instead of using #one to get the book record, then introduce #one! right after that, what if we just jump to using #one! and mention that there's #one also. By default #one! will give them a better error. Especially if we take my propose changes about explaining materializing relations, that'd be a good place to talk about the difference.

Happy to implement any/all of these, just want feedback first.