Refactor: /contributions/stop -> /commit/stop

Our contributions api uses the word commit more
often than not, so let's use that for stopping
contributions as well.
Refactor: Move payment into own module

The code that manages payments and Stripe
wanted to be in its own file.
Refactor: Sort functions by similarity in app.js

Organize app.js a little bit to put similar functions
near each other, and outline the major parts of
the app. This will be useful for further organizing
into components that make sense.