Display Maslow need information in the artefact form

When an artefact has a Maslow need ID, show some basic information about the need in the form. This looks like:
```
As a <role>
I need to <goal>
So that <benefit>
```

To do this, I've added a new method to the Artefact class called `need`, which will make a request to the Need API with the value of the `need_id` field. If `need_id` is blank, if it doesn't look like a Maslow id, or if the request to the API fails, we show a text field instead, just like the current behaviour.

This branch also includes some tidying up of the `artefact_edit_test.rb` integration test so that all the tests based on the `need_id` are together and share similar test data.

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This should be merged around the same time as **alphagov-deployment#364**