Remove charges controller

In preparation to use balanced_payments instead of stripe
Add by_amount scope to levels model
Add donations to donation levels
refactor donations controller
Merge branch 'lea-add-donations-to-donation-levels'

* lea-add-donations-to-donation-levels:
  refactor donations controller
  Add donations to donation levels
  Add by_amount method to levels model
  Remove charges controller

Conflicts:
	app/models/level.rb