Remove render_with method

It provided a little bit of syntactic sugar at the cost of making
controllers and views harder to reason about for newcomers to the
codebase, especially for frontend developers who may not be aware of
where to look for the render_with definition. These developers will be
more familiar with the `render(…, locals: {…})` syntax because it's
commonly used in views to render partials.
Stop using instance variables in views

Replace all use of instance variables in views with explicit calls to
`render` in controllers that pass local variables.
Merge pull request #91 from alphagov/remove-render-with

Remove render_with