Fix register to vote digital take-up table

We were using the default axes, which referenced completion rate.
Merge pull request #639 from alphagov/register-to-vote-digital-takeup

Fix register to vote digital take-up table
Fix bug in line labels on grouped timeseries

There was a bug in line labels as follows: where the ordering of the line
labels was different from the ordering of the models in the collection,
behaviour on mouseover was unpredictable. The wrong labels and lines
were highlighting.

This was because we were finding the selected model based on index,
not on model ID. When the labels were re-ordered by our label
positioning algorithm, the wrong index was being passed into the
collection's selectItem method.

Fix this to find the model by ID instead, and update tests.
Register to vote text changes
Add an additional colour for graphs

We're using grey (from @mikieet) for now because we need a colour.
He's going to have a think about whether this is actually what we
want to do in the long term.
Merge pull request #641 from alphagov/74616598-register-to-vote-changes

Register to vote changes
Singular "Voter registration"
Merge pull request #640 from alphagov/bugfix/line-labels-hover

Fix bug in line labels on grouped timeseries