Define payment observer class
Add missing fields for payment on match

Fields:
  - Key: We send emails to users showing that key, if the user has some problem
  and needs to contact us, they can use that key to identify the payment.
  - Confirmed_at: When we confirm the payment, we set the current date time to
  be able to debug payments using time.
Create shared concern to deal with payable resources

It extracts the code from contribution model and add it to match
Create shared decorator for payable resources

It renames the ContributionDecorator to PayableDecorator and refactor its
specs. Also include the decorator using the shared payable concern.
Move common methods from contribution observer to payment oberser
Add shared notifiable concern to match model
Update rspec-rails version
Refactor contribution observer spec
Rename some contribution notifications to payment
Add missing spec to contribution observer
Use resource to get project name on payment_confirmed_after_finished_project view
Add match association no notifications
Make adjustments on payment notification to work with match
Split contribution observer specs between payment observer
Rename default_state to resource_state on contribution observer spec
Use finalcial intead of credit card on notification
Fix migration name
Show active matches in contributions tab
Render avatar for matches not created by organizations
Calculated already pledged amount of a match