Split `app.rb` apart into multiple `app/*.rb` files

[postmodern]

Split the app.rb file and App class into multiple app/*.rb files.

app.rb:

class App < Sinatra::Base
  ...
end

require './app/db'
require './app/scanning'

app/db.rb:

class App < Sinatra::Base
  get '/db/...' do
    ...
  end
end

[AI-Mozi]

Implemented in #88