Use a tweaked version of the need impact in the view mode

In the view mode, we want to use a slightly rephrased
form of the impact strings so that they fit into
the sentence "If GOV.UK didn't meet this need…".

To achieve this, a new helper method is added to the
need helper which converts the existing impact into
a key which is then looked up in the locale file.
Add a list of organisations at the top of the need

Display all the organisations for a need above the
goal. Each org is separated by a comma and space.
Add met when, justifications, impact and evidence fields to the need

Use the Need's attributes to populate the remaining
bits of data in the need view. 

I've added a second need fixture response to assist
testing when there are empty values returned from
the Need API.

This involves adding some new need helper methods 
to format numbers and percentages in a useful way.