Implement Payable concern for Events
Set up generic payable model
Updated associations, made payable / payment association polymorphic, modified routes

Currently having trouble with form_for with a polymorphic association;
the form_for helper should take an array of [ payable, payment ] but currently
complains of an undefined local variable or method payments_path.
See app/views/payments/_form.html.erb
Fixed borked Gemfile.lock