Simplify Purchase.grouped_payable_by_person

Also write a couple of tests for it.
Removed a lot of duplication in Notifier

Also, added tests for notifier
Tidy up Notifier

* Omit `{|format| format.text }` block
* Only call `subject.deliver` for the relevant tests
Simplify Notifier method generation

I removed the lambda for generating the debt notification methods, since
@koronen pointed out that it was superfluous. I could just use `last_updated_by`
in all of the debt notifications, since it will be set to the same person
anyway.
only iterate on the actions for purchase notifier methods
Merge pull request #121 from datasektionen/improve-gpa

Improve code quality a bit