Simplify method for adding a page/section to the site.

[jmckinnell]

We should try and create a better flow for adding or removing a Page/Section to the site. Currently there are multiple files that need to be modified when creating pages. Something based off the router keys etc might be better.

Relevant files:
https://github.com/Jam3/intern-generator-jam3/blob/master/templates/src/util/async-section-handler.js
https://github.com/Jam3/intern-generator-jam3/blob/master/templates/src/components/Pages/Pages.js
https://github.com/Jam3/intern-generator-jam3/blob/master/templates/src/routes/keys.js

[namniak]

It's 2 files now since we replacedRreact loadable with React lazy. it may rather complicate code and make it less flexible by moving route components from pages into route keys file esp when nested routes will be introduced. though we've done something like that before. open to discussing it further @iranreyes @jmckinnell