Now factories define default headings for investments, so there's no
need to create a group and a heading to create an investment.
Likewise, in order to create a heading it isn't necessary to specify a
group anymore; specifying the budget is enough.
It's possible that there are more similar cases we haven't simplified
yet; I'm only addressing the obvious ones.
The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.
Because of this change, we also need to change `background` to `before`
or else these tests will fail.
